Poker is the quintessential American card games, just about as American as apple pie. Way back before it became a mainstream casino game it was even played by our founding fathers, and another form was played by Native Americans before the arrival of the Europeans.
In the nineteenth century, every state and territory west of the Mississippi River had casinos. And most, if not all, offered poker. Poker was played on the gambling riverboats cruising the Mississippi in Mark Twain's day (just as it is today).
In the late nineteenth century, poker was played in plush casinos in New York and other big eastern cities. When the New York City casinos were shut down, the casinos moved upstate to Saratoga Springs, where poker was played in "Lake Houses," because many of the casinos were located near a lake.
At the turn of the century, the famous gambler Richard Canfield owned the most popular casino in Saratoga Springs, called The Casino. Many of the richest industrial giants of the time played in high stakes poker games at this location (but now you can easily find online poker rooms to play online poker via Internet).
Around 1910, political reform and a progressive movement swept many corrupt local governments out of office and closed down all casinos, so, like alcohol soon after, poker was relegated to the back rooms and "speakeasies" of the day, where it flourished.
There is a Fundamental Theorem of Algebra and a Fundamental Theorem of Calculus. So it's about time to introduce the Fundamental Theorem of Poker. Poker, like all card games, is game of incomplete information, which distinguishes it from board games like chess, backgammon, and checkers, where you can always see what your opponent is doing. If everybody's cards were showing at all times, there would always be a precise, mathematically correct play for each player. Any player who deviated from his correct play would be reducing his mathematical expectation and increasing the expectation of his opponents.
Though this makes poker less attractive for them who normally like to play bingo, roulette or other games which involving more luck than strategy, millions of peoples find if this game fascinating because of the challenging element in making risky decisions only based on a very partial information.
Of course, if all cards were exposed at all times, there wouldn't be game of poker. The art of poker is filling the gaps in the incomplete information provided by your opponent's betting and the exposed cards in open-handed games, and at the same time preventing your opponents from discovering any more than what you want them to know about your hand. Here is a reference for a more complete guide regarding online poker games.