Two differences:

1. If you bust out in a tournament, you have to wait for the next one. And this is also related to payout. You could have busted 30 people yourself in a tournament, and still get 0 dollars. In a cashgame, you can rebuy, and winning pots instantly pays off. So there's a psychological (..and bankroll) difference of dealing with this difference.

2. In terms of game-play, the encompassing difference is stack sizes. TOP tells you how this should affect your play: "All poker starts as a struggle for the antes. If there were no ante, there would be no reason to play." (chapter four)

This ofcourse has many many side-effects on becoming a winning cash game player, but you should understand the fundamentals.

Difference 1 is why I switched from sng to cashgames: I'd double/triple up untill 4 or 5 players left, and then it's basically a crap shoot with enormous variance. And the bad players don't realize this (or even profit from this), so play in the first half is very slow and the variance is even higher overall. That's the beauty of cashgames: you can play actual poker. You don't think squeezing out 5500 players in a week results in the best players do you?