There are three tokens used on the platform. Steem is the speculative currency traded on exchanges that you buy/sell and is created daily from mining done by witnesses that secure the network.
When you deposit steem into your account, you can power up. This upgrades your steem into steem power. Steem power takes 104 weeks(2years) to convert into steem so it encourages long term holding. When someone upvotes your post, you get steem power that the platform encourages you to hold until you cash out in weekly payments and you also get steem dollars, a token pegged to the equivalent of 1USD in Steem. Steem Power is your voting power and the more you have, the more influence your votes on posts have.
This is a basic understanding, but I strongly recommend you check out some of the great youtube videos steemers have posted that explain the currency in more detail, as well as the economics of the system which is also important as some optional but recommended extra learning.