The Button and the Blinds
A button indicates the nominal dealer and this “button” moves round the table after each hand. In reality, the player doesn't actually deal the cards himself - in a casino for instance there will be a house dealer, while in an online poker game, the cards are dealt automatically by a computer. However, to all intents and purposes, the player who is imagined to be the dealer is said to be on the button.
The two players on the left of the button player are required to post the blinds, ie. to make a bet before they examine their cards. The player immediately to the left of the button is referred to as the small blind, and this player puts in half of the lower limit of the game.
For the purposes of illustration we will take a $5/$10 game. This means that all bets in the first two rounds of betting are at the level of $5, and all bets in the last two rounds are at $10. These amounts can be raised and re-raised, but they are the basic units of the betting. So the small blind puts in half the lower limit, $2.50, and the player to his left - the big blind - puts in the lower limit bet, $5.
Thus there is already $7.50 in the pot before the cards have been dealt.