Kass has one goal in life, and that is to make the other team's life a nightmare. Mages will huddle together in fear that you will Riftwalk in from behind them and rip them apart. The team will be endlessly worrying about you suddenly running in and ruining their towers with a lightning fast push.

But most people don't understand how to use Kassadin to his full potential. I see endless builds that use him as a nuke mage. If I wanted to kill at range I would play as Veigar.

Your runes are set up to give you exactly 15% CDR at lv 18 when combined with the 9 percent from your masteries. This allows you to spam your skills to their full potential when combined with a Nashor's Tooth.

Your items revolve around 3 things.
1. Making you move faster.
2. Making your enemy slower.
3. And giving your a powerful DPS component.

Q - Your harassment skill. Level this as fast as you can. But don't bother spamming until it is at least lv 2 as the lv 1 simply isn't worth the mana.
W - Your sustainability skill. A single early level in this skill greatly alleviates your mana issues.
E - Your snare skill. At lv 2 and above, this can be combined with your Q for massive burst potential.
R - Your initiate, escape, position skill. This is a wonderful skill that allows you to drive your opponents crazy with rage as you warp around the battlefield.

Exaust is used to weaken AD champs and serve as your snare while your E is being leveled.
Teleport is used to quickly jump to an ally or enemy turret.

Pro - You are a back door and anti-mage GOD.

Con - AD carries (Xin) will swat you away like a fly. But they do that to most people. You can however, run away really really fast thanks to having an absurd move speed along with your ulti.