Texas Hold'em has become the most popular poker format due to its simplicity and action-packed nature. It's what they used to decide the best in the world at the €10,000 buy-in World Series of Poker main event, and is the most prevalent variant of poker being played today.
In Texas Hold'em, all players are dealt two cards face down (hole cards). There is an initial betting round, which is followed by "The Flop" - three community cards dealt face up in the middle of the table for all players to make use of. A second round of better is then followed by a fourth community card - "The Turn" - being added to the board. A third round of betting takes place, followed by the final community card - "The River". One final betting round takes place, and then the players go to showdown. All remaining players now have seven cards (their two unique hole cards plus the five community cards) to make the best five-card hand with.
Hold'em doesn't use antes, but instead employs 'blinds'. These are two mandatory bets made before the cards are dealt. The small blind is always posted by the player on the immediate left of the dealer, with the big blind being to the immediate left of the small blind. These blinds slowly rotate around the table with each new hand to ensure all players pays them equally.
Though Hold'em can be played as limit and pot-limit, no-limit is the most popular. In limit Hold'em, bet sizes are in set units, and raises an only be made in increments set by whatever the limit is on that table (this is nearly always a set amount for pre-flop and post-flop, followed by a doubling in the bet size for turn and river). In pot-limit games, players can only bet up to the size of the pot at any given point. In no-limit games, you can bet however much you want at any point in the game, right up to every last chip in your stack.
Nope - it's really the simplicity of Hold'em that has made it world-popular.