It's a combination of the two, depending on which ad wins the auction for the slot. They shouldn't be too obtrusive though, I've put them out of the way of the main page content and there aren't any auto-playing video ads, dating ads, or those giant-sized browser-crashers.
For anyone who would like to remove the banner ads from view whilst at the same time supporting the running costs of CC, I've added an account upgrade that is hopefully affordable to most members: https://www.cyclechat.net/account/upgrades - if it's not affordable but you'd still like to support CC's running costs and go banner-free, get in touch and we'll come to some arrangement.
As to ad-blockers, I can't stop anyone using them (and I understand why they have increased in popularity over the years), but if you do choose to block ads please consider creating an extra thread every three or four visits to help entertain and interest your fellow community members, and to grow our Google footprint and attract potential new members.
Of course it goes without saying, but doesn't hurt to say anyway, that your continued support is very much appreciated.
Thanks a lot,