"Part of the genius of poker is its endless capacity for development," explains Al Alvarez in Poker: Bets, Bluffs and Bad Beats, just before going on to list a few of the "literally hundreds of variations" of card games that can be said fall into the poker category.
"Essentially, all these games, even the craziest, are variations of two basic themes: closed or draw poker, in which all the cards are dealt facedown, and stud, in which four of the player's five or seven cards are dealt faceup."
It's a crucial distinction — perhaps the most important one to make among all poker variants, as far as the game's historical evolution is concerned.