The adds are 'by Google' and are (I think) controlled by your own location, browsing & search history etc. In other words, everyone sees different adds. (I think)
I'm getting ads for Progressive insurance...and I think Big Mike is 100 percent correct. The ads are controlled by the cookies that have been left behind by sites you have browsed. Your own computer is "spying on you". Cookies are at work here...I have actually looked for a new insurance carrier recently...and both my MAILBOX and my WEB are aware of that fact.
Yes, I think they are driven by what your browser gleans from your history and what it infers as your interests. Of course, mine shows adds on masterpiece theater, fine art, exquisitely tasteful things in general
It also picks up from keywords in thread titles. While in this thread, you're more likely to see cereal ads. Move to a thread about a cute kitten and you're more likely to see Purina ads.