ELI5 (explain it like I'm 5): What are Steem Witnesses, and what does my vote do?

I've been trying to read up on this but once somebody dives a little too deep into the blockchain terminology it all becomes meaningless.

• What is the job of a Witness?
• Are Witnesses essentially miners?
• How does my upvote effect their ability to be a Witness?
• Do I get any benefits for upvoting Witnesses?

If anybody can help me with this I would be much obliged!