The big and small blinds are literally forced bets. That is to say the two players to the left of the dealer are required to put money into the pot before any cards are dealt. The small blind is placed by the player immediately left of the dealer and is usually half the amount of the big blind,(ie: small blind 50, Big blind 100.) If you are watching poker on tv its probably a tornament and as such the blinds will increase as time passes however if you are playing in a ring game the blinds stay at a fixed level.
