If you're like most of Steemit, the part of every post you pay the most attention to is the little number at the bottom with the dollar sign on it. I believe true Steemit Zen is manifested in being able to ignore that number completely, but until you can do that, it's important to understand where it comes from.
1. Self-voting
In some ways all self-voting counts under this, because you're either spending money or not cashing it out to get your SP. But particularly what I want to focus on here is the practice of leasing Steem Power purely for the purpose of self-voting. Users can buy and sell delegations from places like the MinnowBooster leasing market, and some of those users use their leased SP primarily to upvote themselves. Essentially they're spending STEEM to get more voting power, and then voting to get more rewards.
2. Subscription Services
These are places where you buy in either with liquid currency or with votes, and get votes in return, with a promise of profit. The methodology varies considerably. @steembasicincome offers votes for life for a one-time buyin. @make-a-whale is an account with a group of users who strengthen its voting power by voting for it, and it then returns votes to them. @qurator does some of each, in a very complex system of tiers.
3. Discounters
These are services which act as a clearinghouse for voters wanting to sell their voting power, either directly or through delegation. They give users the opportunity to buy votes for liquid currency at a discount on the value of those votes, with most of the money spent going to the voter, and the service taking a small commission. Not long ago you could tell a discounter because it would advertise a fixed percentage upvote value, but recently a new form has emerged, pioneered by MinnowBooster, that offers a consistent return adjusted dynamically for curation and the price of STEEM and SBD. Services like this range from huge efforts like @minnowbooster and @smartmarket to tiny initiatives like @treeplanter and @earthnation-bot.
4. Bid-bots
A bid-bot conducts a live auction for its vote, usually voting every 2.4 hours when its voting power reaches 100%. Users send money to the bot during its bidding period, and at the end of the period the bot's vote is divided among their posts based on their percentage of the total money sent. There are a lot of these; some notable ones are @buildawhale, @upme, @smartsteem, and @postpromoter. If you're interested in bid-bots, Steem Bot Tracker is an essential resource.
Hybrids
It's also important to recognize that many of the operators and companies behind these services do more than one thing. MinnowBooster is primarily a discounter, but also runs a delegation market. SmartSteem is a bidbot, but the same people run the discounter SmartMarket. Qurator is a subscription service, but they also run the discounter @qustodian exclusively for their members. Sometimes using the same service for multiple forms of vote-buying is beneficial: despite its small limits Qustodian is a significant part of the value of a Qurator membership, and selling votes or delegating to SmartSteem can increase a user's limits for SmartMarket.
My approach
So far I've tried to be neutral in my descriptions, because I think it's important that you have the information to make judgements about purchased votes and the users who use them on your own. And I'm not going to make grand moral statements here, either, I'm just going to talk about how I feel about these options at the moment and why I use the ones that I do.
I use both subscription services and discounters
I'm heavily subscribed to Steem Basic Income; they currently upvote each of my posts about $0.75, and they'll keep doing that forever (in fact it should grow) even if I never give them another Steem or vote. I like that a lot, and I think it's an excellent tool for promoting user commitment and long-term thinking. A lot of contests have been giving away shares as prizes lately, so it's also a really easy service to get into for free.
I'm also a Qurator member, and I use their Qustodian votebot. There's a lot of math involved in figuring out the value of the various Qurator tiers, and it might deserve a full post of its own. For now I'll just say that I see profit in the membership.
I'm a big fan of Minnowbooster. In fact between my last post about them and this one they made me a moderator of their whitelist Discord forum. I'm still just a volunteer, not an employee, but I'm there a lot because I see a lot of value in their service. I think allowing users to sell their voting power when they're unable to use it themselves is absolutely essential to maintaining a system where Steem Power is locked up for long periods of time. And of course I'm a fan of the way they allow me to buy those votes at a substantial profit for myself.
I also use Smartmarket. They've had a bunch of technical difficulties during the last few weeks that make them less appealing than they were when I started investigating vote-selling services, but I like their developer and I hope he'll get things straightened out again soon.
I'm not eligible to use Earthnation-bot, but if you're a tiny user you should investigate it. It's the most profitable voting service on Steemit, albeit with tiny limits.
I don't self-vote, and I don't use bid-bots
In fact I've decided to stop even the regular form of self-voting, even though it will reduce my post rewards. I'm not here to tell you it's wrong to self-vote, because I believe strongly that the system only works at all as long as people are allowed to use their SP as they see fit. (You will find self-voting in my comment section to be counterproductive, however.)
I am going to tell you one different reason not to self-vote, though: you are missing all the fun. Being able to wantonly fling small amounts of money at people because I like their work is the best part of being here. I'm doing all of these other things so that I can fling larger amounts of money at more of them. Because it is great.
When it comes to bid-bots, I really dislike the auctioning system that they use, and I think on average they're predatory on their users. At best they're like video poker: possible to make money from by thoroughly understanding the system and being extremely disciplined in your decision-making. Which is something few users do, and even personally, I'm not convinced that learning to profit from them would be worth my time with all of these other options available.
I'd be interested to see some other auction approaches investigated by bot operators. There are many types of combinatorial auctions out there, and the state of the art on Steemit is pretty much the worst one unless your only priority is bot-owner profit.
Conclusion
I hope I've helped your understanding of how things work on Steemit, and I'm happy to answer any followup questions in the comments. This is a subject on which a lot of users have difficulty treating each other respectfully, so I'd like to ask you to be intentional and friendly about your commenting.