Payouts will begin on July 4th. After that point you will be able to cashout once the last upvote is 24h old.
The posts are funded by newly created STEEM on the blockchain. It is pretty much the same thing as in Bitcoin, just that the blockchain doesn't pay miners but people to participate in

• content writing
• voting
• market making
• and more

you can find the documentation on steem.io.

Note, that the amounts stated next to the thread are split among the post author and those that voted as participated in the discussion.