It is an enchanted physical object (often a jewelry piece) that must be attuned to you with Karma (see Bonding p191), then carried on your person to be used. Rules for creating them can be found in the Street Magic supplement.
Acquiring a Focus is just as easy as acquiring Weapons, which is to say they are expensive and legally restricted, but since when has that ever stopped your character?

The availability and cost of a particular Focus (as well as it's Karma cost to attune/bond) is based on it's Rating, per the chart on p.340.
The Rating of a focus is also how many dice it will grant you on the tests it modifies. Note that Power and Weapon focus are the only two focus types that are general, all others must be specific to a specialty. That is to say, you cannot purchase a Summoning Focus, you must purchase Summoning (Spirits of Man) or Summoning (Spirits of Air). See the focus descriptions for Spell and Spirit foci on p.191 for more information.
A weapon focus is a bit different from the other foci as it does not add to Magic Tests. Instead, it adds to Melee Combat tests when using the weapon focus, which is literally an enchanted weapon, traditionally a sword of some type though any weapon type can be turned into a Weapon Focus. You could even technically turn an Assault Rifle into a Weapon Focus, but you would only get the bonus dice if you were using it as a club and beating someone over the head (Melee tests, remember). The other advantage of a Weapon Focus is that it is the only type of weapon you can carry into Astral Space. All of your Active Foci will be available to you when you are projecting Astrally.
Finally, Sustaining Foci are worthy of specific mention as these are a must have for anyone who hopes to keep a spell constantly active. Normally, Sustaining a spell gives you a 2-die penalty to everything else you are doing, as sustaining is a very distracting activity. A Sustaining focus - which must be Sustaining (Health), Sustaining (Manipulation), etc - can sustain a single spell with force less than or equal to its Rating, so that you do not take the 2-die penalty. I encourage any mage to consider acquiring a R4 Sustaining (Health) Focus and the Increase Reflexes spell as soon as possible.

Finally, it is worth noting that Foci are active on the Astral Plane as well, and are very noticible to Astral Perception. Also as a result, it is difficult to carry them through Mana Barriers (see pp.185-186, Passing Through Barriers). How often those barriers will be encountered is entirely up to your gamemaster of course.