In regular physical combat, you normally have 3 defensive options
Fists, which can defend other unarmed attacks
Weapons, which can defend against other Fist and Weapon based attacks
and
Athletics, which can generally defend against anything
Very rarely (mostly because it's actually quite powerful) an attack will require an alternate skill to defend against. For example, earth magic evocation used to cause earthquakes and similar tricks are tagged as defended against with might. (Or, we'd assume, just finding some way to get off the ground.)
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Social and Mental combat have different defensive skills (Usually rapport for social combat and usually discipline for mental combat) See the appropriate sections of the book for them.
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The defenses are just normal skill checks (e.g. roll the 4 fudge dice and add the result to your skill). Note that ambushes can reduce your effective skill to 0, but you still get to roll the 4 fudge dice and add.
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When people refer to 'blocks' as a defense against attacks, they are almost always referring to defensive blocks put up by evocation (or enchanted items). Note that evocation blocks aren't 'rolled' after they are put up, you just use their fixed value.
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You get 2 focus items slots from any of the following:
ritual
thaumaturgy
channeling
evocation
You normally get 4 focus item slots from sponsored magic (which is really ritual + channeling + some special effects). However, if you already have full thaumaturgy and/or full evocation, the amount of 'free' focus item slots you get from sponsored magic is reduced. (i.e. you can only ever get 4 free focus item slots).
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You can also spend refresh on 'refinement' which, among other things, can get you 2 more focus item slots.
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You can trade out a focus slot for two enchanted item slots.
So, for example, a basic -7 wizard (the sight, evocation, thaumatrugy) gets 4 focus item slots.
He could then trade out 2 of his focus item slots from 4 enchanted slots.
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Foci don't have limited uses, they just power up you thaumaturgy or evocation when you have them. It's enchanted items that have a limited number of uses.
The basic enchanted item starts with a strength equal to your lore, and has 1 use per game session.
You can increase the base power and/or number of uses by taking crafting related thaumaturgy specialties, having crafting related foci, or by using more than a single enchanted item slot on a single item.
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Other enchanted item notes:
It's frowned upon to make an enchanted item with more power than twice your lore.
You can gain an additional use out of an enchanted item by paying one mental stress at the time of the use.
Enchanted defensive items cost no action to use and can generate either a block at their base power, or armor at half-power (round down).