With STEEM you get paid for your original content for example you grow organic pineapples in bat shit and you would like to share it with the world. Now I read about your pineapples and I like it. So I upvote it. Upvote = Me sending you STEEM for posting a good article on organic pineapples. Then you go and look at who sent you STEEM (dropdown next to votes) and you click on their profile links. The articles you like and upvote returns the favor so we have just completed a STEEM transaction.
Steem Up -> Take your rewards and turn them into steem power (SP). Now you are permanently vested with the STEEM network and you can reep reward from all the features Voting/Blogging/Interacting/Trading.
Trading -> Found under Market in the Hamburger Menu Button
Power Down -> You can cash out by powering down your SP. Be warned that you will get paid weekly over period of 104 weeks