resteem equates more or less "share" in FB or "retweet" in Twitter.
upvote is similar to "like" in FB. however there is the reward system attached. the more upvotes you get and the more steem power the upvoting people have, the more steem you get paid 7 days after your post.
When you upvote yourself, you can also get steem for it (curation reward) , especially when you are an early upvoter and if yiu have a lot of steem power.