What Is Video Poker?

It is a casino game based on five-card draw poker rules.

Video poker is essentially quite similar to slot machines found in casinos except that as Tessastua mentioned it is based on five-card draw poker. In very simple terms the gameplay usually consists of you making a bet and the machine will then deal you a hand. You will then get a chance to discard one ore more cards and draw new ones. Afterwards the machine will evaluate your hand and give you a payout if you have a winning hand.
Winning hands will be listed on the machine and may differ but usually the lowest possible hand you need for a payout is a pair of jacks. Usually the rarer a particular hand is the greater the payout will be.

If you'd like more details I'd suggest to read this wikipedia article and I also came across this site on video poker which has a fair deal of information on the matter and lastly here's a detailed step-by-step instruction from an online casino on how to play video poker which I think does a good job of giving you an idea of the gameplay.

Hope that helps.