Pretend you're pitching in a baseball game. The hitter comes to the plate, and, not knowing what kind of hitter that player is, you throw her a fastball. When she hits a double, you get to send her back home to redo the at-bat, giving her a slider this time.
Essentially, that's what Words with Friends lets you do every turn. In Scrabble, putting down your tiles to make a word means you're done. The word can be challenged and looked up; if it's not in the dictionary (or a Scrabble-approved word list), you're out of luck -- and a turn.
When playing WWF, you're given the option of putting a word down and submitting it. If the word isn't on the approved list, WWF asks you to choose another valid word (very much unlike your cutthroat Scrabble opponent, who would pounce on your mistake and make you pay). This means you can submit combination after combination until you happen to stumble upon a word like "eagre" (a tidal flood), thus impressing your friends with your lexical grasp.