Exchanging links with thousands of irrelevant sites. Yes, it affects keyword ranking greatly, you can be on the first page of a large scale keyword within 2 months, but once google finds out about your little link farm, it will ban you forever. That why you shouldn't do it. If you exchange links with a link farm and link to them its bad for your site, and their inbound link is worthless if of course they are a confirmed link farm.