Illusion is basically the crowd control magic school in Skyrim. It is a great support school, but is not a very effective school to use as your primary, although it can be done.

Let's break down the skills and their uses.

1. Clairvoyance: Shows you where to go, pretty obvious.

2. Courage/Rally/Call to Arms: This is good if you roll around with followers or conjured Atranochs/zombies. While the "not running away" part isn't that big of a deal, each rank does give good stat boosts to your little army. That's where these spells excel.

3. Fury/Frenzy/Mayhem: These are good if you come into a room full of mobs(enemies), and is even better when you are not seen. Just Fury/Frenzy one at a time till you have a manageable number left, or just Mayhem the whole pack and watch the ensuing mayhem. Good way to clear out groups with minimal effort, while remaining out of combat and/or sight.

4. Calm/Pacify/Harmony: Makes mobs not attack/stop attacking you, very useful for someone who uses sneak a lot.

5. Fear/Rout/Hystaria: These spells make mobs stop attacking you and run away. Very useful, between this and the calm spells, these are your main ways to keep mobs out of the battle so you can take them down much easier one at a time. Pull one more mob then you can handle? Fear him and take out the others first, then come back once the fear breaks.

6. Muffle: Quiet footsteps make you more sneaky, pretty self explanatory.

7. Invisibility: Makes you invisible, when used with muffle, pretty much makes you undetectable till you attack or use something.

That covers all the spells, hope this helped some. Just remember not to rely on this school as your main, but when mixed in with any hybrid character, Illusion can be very fun/entertaining and quite helpful in many different pinches.