Adding to @MainMa answer..
They Trick others to into doing the CAPTCHA for them
Basically they set up a Wearz site or a porn site the has a CAPTCHA on it, But its not a real CAPTCHA.. A bot pulls the CAPTCHA from the site they want to spam (or whatever) then displays it on the Wearz site or a pron site where someone completes it for them. Then the CAPTCHA value is passed back to their bot...
A bit more on Spammers..
I use reCAPTCHA, its basically worthless, I also have a custom spam filter that catches the spam (that got passed reCAPTCHA), and I need to review it every few days for false positives.
My forum is also all custom written and gets very little traffic, no-one coded a specific attack to my site.
My spam filter catches 2k spam messages a day! Non are ever displayed on the site, they get no benefit from spamming me yet they still do.
I can see the patterns in the spamming attempts because I log it all, I can tell you this: putting aside how they get past the CAPTCHA, they are clearly using a brute force technique varying what fields are filled out and by what kind of data and word mixes populate those fields. Apparently they do this so cheaply (including bypassing the CAPTCHA)
That it doesn't even pay to do an analysis of the individual sites to see of if what they are doing is or isn't working.
They keep targeting my site for years on end with 1000s of spam messages a day only to get one through month, and that one gets manually deleted a day later. It's that cheap to spam!
This is going to be a battle for years to come. Particularly for small one man moderator sites like mine.