The Powerup abilities are as follows:

More Cards:
Adds more cards to your deck so you can continue your level. This PowerUp will only appear if you run out of Deck cards.

Wild Card:
Using this PowerUp will make a blank letter tile appear in the word bank, which will become a valid letter if applicable.

Triple Word: 
The next word you submit earns you triple the points!

Destroy Card: 
Will remove any selected card from the Board. This PowerUp cannot destroy non-letter and special attribute cards, and any cards removed this way will not award points.