Poker is a game with many versions, each using a unique set of rules that differentiate it from the others. Compare Texas hold'em with five-card draw: in hold'em, each player receives two hole cards and combines them with three of five community cards to make a hand; in draw, each player gets five private cards, swaps some out for new cards, then reveals their hand after the last round of betting.
Numerous iterations of poker have exclusive elements, but all include a core set of principles that define them as forms of one game, rather than a set of unrelated ones.