Video poker games and slot machine games look a lot alike. They're both coin-operated gambling machines that a player sits in front of and pushes a button (or buttons) or pulls a lever to operate. Both machines have a hopper that pays out a players' winnings in coins, although newer machines print out a ticket instead of spitting out coins. Either way, the games are similar in appearance and have the same purpose: to allow gamblers to play without having to interact with a dealer or with other players.
Modern slot machines and video poker machines operate using a random number to generate their results. This random number generator (RNG) is a computer chip that is constantly generating a series of random numbers, even when the machine is not being played. When a player makes a wager, the player receives whatever number the machine is cycling on at that minute. In some ways, the random number generator is like a constantly spinning wheel of fortune which stops instantly when a player makes a wager.
The main difference between video poker and slot machines lies within the random number generator. On a slot machine, the random number generator is programmed to pay out a percentage of the money that's run through it at a given percentage, usually anywhere from 83% to 97% over time. That amount is called the payback percentage, and it's completely determined by the settings on the RNG. There is no way to determine what a slot machine's payout percentage is just by looking at it, and to determine it by the machines payouts would require 100's of 1000's of trials.
On a video poker machine though, the random number generator duplicates a deck of 52 (or in some games 53) playing cards. The odds of getting any card is the same as it would be if someone were dealing from an actual deck. Since the payouts are determined by what hands a player receives, and since someone can mathematically determine the odds of getting each hand, a person can relatively easily calculate the payback percentage of a video poker machine.