It's different because in limit, the betting is more free, and you can raise and re-raise until the cap several times. In no-limit, it's easier to tip off your hand's value with an overly large raise, so if your opponent thinks they have the best hand (or is bluffing at it), you can simply call and let them try to scare you out of the pot. If you re-raise, you let them know you have a very good hand, which might cause them to fold, meaning less money for you at the showdown.