You're definitely right that Steemit is pretty confusing for new members, I still haven't really got a clue what the difference is between good bots and bad ones.
The spam comments annoy me too, I don't have too much comments on my posts in general, but reading through other people's comment section I can see that spam is pretty rampant.
A bot is any account that is automated to do certain things. Bad ones are those that aim to gather maximum attention/followers by simply posting random generic replies to hundreds of post per day.
Then there are vote-bidding bots, that will upvote your post if you send them SBD. A lot of them seem to be uncurated, but the bigger ones are curated. (to avoid 1pic posts to be upvoted. We want legit posts to earn more not spam) Good bots usually have a visible team behind them. They specialize in curating original content, supporting newcomers or fighting plagiarism. They have a post explaining their mission and post weekly updates etc. Like @cheetah @curie @minnowsupport etc
You can make your own account into a bot too via steemvoter.com - it will automatically upvote authors you want upvoted with your account.
I also suggest installing this addon for chrome/firefox https://chrome.google.com/webstore/detail/steemit-more-info/dcbpmclnlapbkgkddhencielibcjogcf
OMG, I forgot Steemvoter after setting the voting "rules". I was trying hard to find out, with what Streemian configuration I was auto voting. Whoa, it's Steemvoter and not Streemian.