So let's see if I got the Grid thing strait first:

(Now edited from posted replies.)

Grids are the term for matrix providers. They are not only how someone connects to the matrix, but they are also the the particular groups on which one setups devices or hosts. Every device and host is on a particular grid, and their is a -2 penalty for attempting to take matrix actions on something on a different grid than you. This -2 penalty for cross-grid traffic is not defined as Noise, despite being described as Lag, but is a direct penalty to the dicepool.

There can might also be outright grid restrictions. Some hosts in particular might only be accessible to people on the same grid, or they may exclude people from particular grids (most often people from the Public Grid.) According to a comment on page 221, hosts might be able to restrict Public Grid users from "legally" accessing their Host, and therefore it might be possible in broader contexts like Aztech not letting anyone on another global grid (like a competitor megacrop) access their test facility's host, but in general a Host is said to be accessible from all Grids, although cross grid penalties apply. Additionally, each Grid has its own demiGODs. Thus, the NeoNet Grid has NeoNet employed demiGOD agents keeping their matrix safe. It's implied that demiGODs from various grids are competitive or even hostile with each other, but still share over-watch scores for hackers with each other in real time, so there is effectively little difference between Grids from a game mechanics standpoint.

There is a Public Grid which is crappy, but free to all. Anyone and everything can connect to it unless you're in a place with too much noise (like wifi inhibiting areas, too much cross traffic, or in the middle of no where without a satellite up-link) Using the Public Grid inflicts a -2 penalty on all matrix actions, and this is not defined as a Noise penalty, so it applies to all matrix actions and can't be reduced through Noise Reduction. Also, many Hosts simply do might not allow access to people on the public grid. Lastly, the -2 Penalty for being on the Public Grid even applies to actions taken inside a Host.

Next up are the Local Grids, which are region based. Seattle has it's own grid, for example. If you're lifestyle is Middle, you have a public grid connection. We can assume that most local devices and hosts are running on the local grid if they are semi legitimate in nature. The vending machine might be on the public grid, but the local Bank is not. To connect to a local grid, you must be within it's coverage region. That means anyone attempting to hack a Seattle Bank will be taking a -2 penalty unless they are on the Seattle Grid, and therefore in Seattle.

Lastly there are global grids, 10 of them, one for each megacorp. You can connect to these grids just about anywhere unless the noise penalty where you stand sucks (IE: middle of the dessert without a satellite up-link or inside a bunker with wifi inhibiting wallpaper, under a few feet of salt water, etc.) If your lifestyle is High, you get to pick a global grid provider. It can be assumed that each mega-corp would normally be keeping their own devices, people, and hosts on their own grid for the obvious reason of getting to use their own security demiGODs, etc. That also seems to mean that if you want to hack a NeoNet research facility host in Seattle, you can do so from Berlin without a cross grid penalty as long as you are on the NeoNet grid.

Distance Noise
The 4th edition concept of Signal isn't a trait anymore, although for the purposes of getting a wireless bonus on some gear, Device Rating is involved. Instead the Noise rules handle things with a flat scale for everyone, with various gear (RCCs for example) providing noise reduction instead of stronger signal. That means a Berlin Hacker attempting to break into a Seattle Bank device would not only have issues with the cross grid penalty, but also a distance based penalty for being more than 100km away (that's a noise penalty of 8 or 5 with a Satellite up-link.) Hosts, however, do not have a distance penalty, so if a Seattle Bank has slaved all it's devices to a Host in order to better protect them with IC and a shared firewall, they also reduce distance noise penalties to 0... something both customers and hackers must love. This also means that if you are on a run against a NeoNet facility using a host for their matrix security, and you hop/hack onto the NeoNet grid, you can hack that facility from across the globe with no distance noise penalty or cross grid penalty. However, any devices in that facility not slaved to the Host will have a massive noise penalty for distance, which makes it useful to be near by.

For a Decker:
So between distance noise and cross-grid penalties, you generally want to be near the target of your hack, and standing in a place that doesn't have terrible spam or static (IE: location based noise blocking your access to the matrix.) If you're going to hack a Seattle Bank Device, you want to be within 100 meters to avoid a noise penalty for distance and you want to be connected to the Seattle Grid to avoid a cross grid penalty. Now there are a few ways to reduce noise, including a wifi enabled datajack and cyberprogram, so in theory you could hack from up to 10 kms away and still scrub 3 points of noise and still be in the local grid coverage zone, but you're giving up a program slot on your cyberdeck to do so.

A major concern for grid hopping deckers is that hopping onto the grid of your target will likely require an illegal Attack or Sleaze check, and that can start up your Grid Overwatch Score. Once you have a score of 1 or higher, it starts to accumulate by 2d6 every 15 minutes, so time becomes an issue.

Of course, just as in 4th edition, many runs will feature devices or spaces that are not directly connected to the matrix thanks to Wifi Inhibiting wallpaper, "throwback" jack only systems, etc. For those, the Decker will need to enter the complex with the rest of his crew, same as it ever was.

For General Shadowrunners:
If all you are interested in doing is getting your Wireless Bonus on your gear, say that +2 dicepool bonus for using a Smartlink, you just need to be connected to the matrix. In this case the Public Grid will work fine as the -2 penalty is on Matrix actions, and shooting people with a gun isn't a matrix action. Your main concern becomes local noise. If the Noise is higher than the Device Rating, you don't get your Wireless bonus. Generic device ratings are listed on the top of page 421, but most starting gear for a shadowrunner will be in the 2-3 range, meaning it's actually very vulnerable to jamming, spamzones, and static zones when compared to the example Noise chart on page 231 or the jamming action on page 240. Noise Reduction doesn't seem to be a "defensive test", so slaving your devices to a Commlink (which can easily be in the 4-6 device range at character creation) doesn't help with noise, although it still helps with with basic security. Confusingly, Noise inhibits the wireless bonuses on devices where the noise is greater than the device rating, but on matrix actions, like running a matrix search, it only applies a penalty. This theoretically means you can always send a message (a matrix action with no dice rolling) even in the worst static zones, although more realistically you should be limited by your link's device rating vs noise.
If you're really worried about Noise (maybe because you're very good at matrix research) the only real options you have are to get a Datajack, which has Noise Reduction as wifi bonus (try not to think about how the implant has a device rating of 2 so in order to get noise reduction wifi bonus you need to have low noise present), and/or to buy an RCC or Deck. Note, an RCC is cheaper than a Deck, and has better Noise canceling options.

For Riggers
Rigger Control Consoles have Noise reduction features that can make it possible to maintain a connection to and alleviate noise penalties from using drones. Additionally, Riggers have a specific noise reduction skill use for Electronic Warefare (see page 268), and can use the Noise Reduction cyberprogram. In most cases your drones are slaved to your RCC, which is itself part of your Persona, and therefore all your drones use whatever Grid you are on. Drones not slaved to your RCC are probably on what ever Grid you connected them to, which is any grid you had access to when they were placed onto the matrix, and could theoretically suffer from a cross grid penalty if you've hopped grids since then. This also means the Public Grid, with it's -2 matrix action penalties, could be trouble for Riggers. However, the wording on page 266 says "The control rig allows you to treat Vehicle actions the same way you treat Matrix actions, so any bonus you get to Matrix actions also apply to Vehicle actions when you're jumped in," which might mean only bonuses (like better matrix initative) apply but not penalties (like a -2 for using the public grid) applies... but don't blame me if your GM hits you with the core rule book for making that argument. Either way, Control Device is a matrix action, so controlling drones instead of jumping in to rig them does suffer from being on the public grid. Additionally, because you lack an Attack or Sleeze rating on your RCC, you can't just hack your way onto the local grid to run your drones more efficiently. Riggers will need to either have a Middle lifestyle to get onto the local grid, or buy access to it, which sadly doesn't seem to have any prices or details in the core book beyond the note on page 222's sidebar. Thankfully, a direct connection to a drone or vehicle, like say plugging into your car with the retractable 1 meter cable built into your Control Rig (that thing implanted in your skull), has no noise penalties or grid issues.

For Technomancers
I honestly haven't looked in any detail at Technomancers yet. I do know they have a complex form that connects them to all grids at once, alleviating cross grid and public grid penalties. I also know a number of their other Complex forms interact with Noise, and that they take Noise as a penalty on Threading their complex forms.