You pay what you like but have to pay at least $1 (60p~) to get Steam keys and you receive the basic set of games they offer from the bundle. If you pay over the average amount (which is an average based on what everyone has paid, currently $7.99 or £4.67~) then you receive bonus games. If you pay over the pre-set $20 amount you receive the highest tier games.
You can split what you pay between the developer, charity and Humble themselves, such as you can give it all to charity if you wish, or all to the developers, or a third each etc. When you buy the bundle you will need to link your Steam account to your Humble account so you can register the games to yourself. You can also create gifts from the games you have purchased and give them away to friends.