It's a social network, where you share content and interact with people. Just like Facebook or Twitter. The differences are:

• this is decentralized - it's not owned by a single entity, although it was created by a legit business
• for every interaction you get a reward, in the form of a cryptocurrency
• there are 3 types of currencies: steem, steem power and steem backed dollar (the savings account were introduced only recently and they are there to protect your funds, if you want to withdraw from there it will happen over 3 days - it's still unclear for me too)

Now here's where is getting interesting:

• steem, as a cryptocoin, can be traded in exchanges and more steems are "printed" everyday, to create the supply needed for the interactions
• steem power gives you "authority" in the social network: the more SP you have, the more "money" you direct to the content you upvote. It's very important to know that SP cannot be converted directly into steem. You can convert into steem only by a process of 104 weekly payments, during 2 years (it's called "powering down" - and when you convert steem into SP it's called "powering up"). So SP isn't very liquid. As a matter of fact, it's more like a share you own in the company itself.
• steem backed dollar is actually a smart contract, it's pegged to the USD, which means it will always hold $1 worth of steem. SBD also earns 10% interest if you just keep it into your account. You can only convert SBD back into steem by a 7 days interval, meaning the actual transaction will take place 7 days from the moment of initiation.

So, every time you post something you get paid in a combination of steem, SP and SBD. You get paid if you upvote something, especially if you're among the first to vote and if that post becomes popular. It's called "curating reward" and it's a way to reward people who are filtering valuable content from garbage. The curation reward gets bigger if the entire pool of the post is bigger. Ex: a post is listed to be worth $100. Out of this $75 can go to the author and $25 to all the people who upvoted the post. And out of the $75, you get $35 in SP, $17.5 in steem and $17.5 in SBD (these are not exact numbers, just a simulation).

You can apply different strategies based on your goals. If you want liquidity, convert everything into steem and change them into an exchange in your preferred cryptocurrency and then to fiat. If you want to stay in the Steemit community a la long, you should then power up a large portion of your reserves, to acquire SP, in order to grow your influence.

This combination makes Steem unique and as a beginner myself (I'm here for less than 10 days) I look forward to see it unfolding.

P.S. Oh, and one funny and cute thing: the levels of richness are measured in the ecosystem based on marine animals: so people who own the most value are called "whales", the next layer "dolphins" and so on...