Same dictionary as Words With Friends.
Longer words give you better bonus letters, and more time.
4 letters = Double Letter
5 letters = Triple Letter
6 letters = Double Word
7 letters = Triple Word
8+ = All four!
Re-tap a letter to un-select it, and any letters you'd selected after it.
Each level gives you one of each bonus letter, requires more points to pass, and the timer runs a little faster.