Welcome to Steemit, Internet! Let me show ya around.
First of all, you may be here because you saw a blurb on the news or someone mention it on Facebook, but you're not really sure how it works, or how cryptocurriencies work, or any of that. No worries! I didn't either when I first started. So here's a brief primer.
How Payments Work
You're probably here because you heard "get paid for the time you spend on social media," so let's start with payments. The dollar amount you see under posts isn't US Dollars, even though it uses the "$" symbol. It's the amount of potential (because it can change during the week that voting is open on it, more on that later) earnings in the native cryptos, Steem and Steem Backed Dollars. Further, it is split between the author of the post getting most of that payment, and the curators (the people who voted for it) getting a smaller portion in relation to how big their vote on it was. If you click on the little down arrow to the right of the number (the up arrow in a circle to the left is what you click to upvote), you can see the "pending payout," which will tell you how much of it will be SBD and how much is Steem (it will say SP for "Steem Power").
Even though there are only three settings as to how you want to receive your rewards (100% Steem Power, 50/50 SP/SBD, or decline payment - you'll see the numbers crossed out if the author has declined any payment), on a 50/50 post, the numbers will be uneven. Why? Because the system presumes SBDs are worth one US dollar (tl;dr they used to be always worth one dollar, now their value fluctuates), but it calculates Steem Power based on the current price of Steem on coin exchanges. So usually that means you'll get more SBDs than SP.
Still with me? Okay.
Voting on a post lasts for seven days from the time of publication. Really, six and a half; the last twelve hours won't add any money if you upvote it. It can flux depending on more people upvoting it, someone flagging it (essentially, a downvote), and the price of Steem. How big of an upvote you can give is dependent on your Steem Power.
Okay, explain Steem Power to me
Steem Power is Steem (the currency) essentially locked away in a retirement account. You CAN cash it out, but not all at once (it will take 13 weeks to get all of your SP made available, divided into once weekly payments). If you want to grow on the platform, though (and earn bigger curation rewards - that portion of payments that goes to upvoters), you want to leave your SP in that account. It effects the size of your upvotes, your curation rewards, your rank, and how much attention others give you. Personally, I treat my SBD as "money" to pay bills with, and SP like an automatic contribution to a retirement account. I leave it there.
So how does flagging/downvoting work?
Unlike other sites, there is no centralized body who can banhammer users, delete their accounts, or deny their earnings (coughYouTubecough) if someone reports their post for some reason. The upside, of course, is you can't be censored. Breastfeeding mamas and burlesque dancers don't need to worry about Facebook jail because someone with a stick up their butt reported them as "inappropriate." The downside, is if there is really a creep, no one can boot them, either. HOWEVER, they can be downvoted into obscurity.
Say there is some spam account who just posts 500 comments a day of "Nice post, follow me!" (they do exist); very quickly people will flag those comments, dropping their reputation like a rock. Once their reputation is low enough, they are automatically "grayed out" - you have to click to see what they said. Easily ignored (and not profitable for them).
What's reputation?
That little number in parenthesis next to the person's name. It's supposed to be an indicator of, essentially, earning your reputation as a good account on the platform; people upvote you because they like what you have to say. However, for complicated reasons I won't get into here (there's plenty of posts ranting about it if you look), some people have essentially bought their rep, so the system isn't perfect. Everyone starts at reputation 25. Once you start going below that, you automatically look suspicious (with the exception of well known accounts with negative reputations; again, it's basically due to site politics and arguments about the unfairness of bought votes and gaming the system).
How do I turn my Steem Dollars into US Dollars that I can pay the electric bill with?
There are many detailed tutorials taking you step by step through the process depending on what route you'd like to take, but basically:
Step one: using an exchange like Blocktrades or Bittrex, you change your SBD into one of the "big" cryptos, like Bitcoin, Etherium, or Litecoin. You deposit those coins into a wallet at a site like Coinbase.
Step two: At Coinbase, you sell your Bitcoin/ETH/LTC for dollars, which will deposit into your bank account.
There are lots of other exchanges and wallets out there; you don't have to use Coinbase, Blocktrades, or Bittrex. I was just using them as examples. People like other exchanges for many reasons, like lower fees. But Coinbase is one of the big guns, so to speak, and pretty easy to navigate (and find tutorials for). Poke around to find the exchanges you prefer!
Congratulations! You just made it through the most complicated part of this primer. Give your brain a high five.
Geez, is it all about money?
Nope! One of my favorite things about Steemit are all the people trying to make the world a better place. I love the crew at #ophumanangels, where we post about little acts of kindness and encouragement; #litterwalk is where we post about our litter-cleaning missions; and there are at least three tree-planting accounts that I know of: 
There's lots of artists sharing their work, visual, musical, and written; more contests than you can shake a stick at, and of course, allllllllllll the cryptocurrency news. We even have #steembay - selling your goods on the blockchain!
You should think of Steemit as less like Facebook, and more like a true internet of ideas.
BUT I STILL DON'T KNOW WHAT BLOCKCHAIN IS, PHOENIX!
I know. Some dude on cable news mocked it, trying to sound smart instead of admitting that he didn't understand it, either. "It uses so much electricity!!! It's bad for the environment!!" ...those numbers are actually based on older tech used to mine Bitcoin. Most Bitcoin mining has moved on to a more efficient system now, and not all cryptos use the same mining tech as Bitcoin, anyway. How much electric does Google use? Facebook? YouTube? Wall Street? You get the idea. It's not the eco-apocalypse.
"But BAD ACTORS use cryptocurriencies, like DRUG DEALERS on that one site!!" ...that site was shut down ages ago, and uh, drug dealers use cash, too. You gonna boycott cash? No. That's a silly argument.
So, you wanna understand blockchain? Here are my two recommendations. One, the book The Age of Cryptocurrency (link to it at a mom and pop glbt-friendly bookstore, but you can also find it at The Behemoth That Shall Not Be Named) by Paul Vigna and Michael Casey.
Two, this excellent YouTube video from SciShow:
not at all my work, but it's totally what I point people to all the time
I hope this post helped you feel a little less lost trying to figure out what exactly is happening here in Steemit land! If you're new, don't forget to make an introduction post, and put the tags #introduceyourself and #introducemyself in it! Thanks for reading, and...
WELCOME TO STEEMIT!
That Red Fish your momma always warned you about
Referral links:
Get free Manna, crypto UBI: https://www.mannabase.com/?ref=6e4b50d35e
Join Simbi, the barter economy website: https://simbi.com/wren-paasch/welcome