Google, Bing, AOL, Facebook, and other sites like these are considered “tier one” networks because they provide high-quality traffic (“high-quality” meaning that they are humans rather than robots, or “fake” traffic) and clicks by people who have seen an ad and made a conscious decision to click it. These are ultimately the people you really want on your site.
There are also, however, “tier two” providers and remnant ad providers. I'm mentioning them because they are a viable source of traffic and can be very cheap (half a cent or so per click), which is why many Web publishers and sites use them. But the traffic they send can be questionable—much of it is simply non-human, robot traffic—and the way they send it can be a bit…sketchy (you don't want to know), so it's not a good source for you to rely on for anything other than to get some small baseline of traffic going. You can do a Google search for tier two and remnant ad networks to find ones you may want to use. I won't list them here, as they're not a source of traffic that should really be promoted. But in certain situations, if you need some cheap traffic to meet a specific need, it's a resource for you to consider.
When it comes to marketing, there's virtually no end to the various avenues you can choose from and tools you can take advantage of. That's what makes marketing so effective, exciting, and confusing! There are far too many options to go deeply into in the remainder of this book, but I did just want to touch on a couple more that I think are important to mention.