"Poker" is really a family of games with different rules for dealing cards and forming hands. Here are four formats you're most likely to come across.
Texas Hold'em
The most recognizable format: each player is dealt two hole cards, and five more community cards are placed on the table. Its simple rules combined with deep strategy have made it the standard for tournaments, including the World Series of Poker.
Omaha
Similar to Hold'em, but players receive four hole cards and must use exactly two of them together with three of the five community cards to form their hand. This increases the number of possible combinations and makes the game more layered — Pot-Limit Omaha (PLO) is an especially popular variant.
Seven Card Stud
A format with no community cards: each player receives seven cards (some face-up, some face-down) and forms the best possible five-card hand. It's considered one of the oldest formats — before Hold'em's explosive rise in popularity, Stud was the standard game in home games and casinos alike.
Razz
A "reverse" variant of Stud, where the lowest hand wins instead of the highest (a lowball game). It's an unusual format that requires you to rethink your usual sense of hand strength.
Where to start
If you're a beginner, start with Texas Hold'em: it has the most learning materials, simulators, and platforms with low-limit tables, where you can practice without putting serious strain on your bankroll.