Link farming is biscally the the way where we get back links from that sites which have bad impact on search engines like If you have a real Estate site for SEO and you get back links form porn site casino sites etc that will be called link farming.
Link farming- website are links to each other randomly without any category, only for the sake of link.
A link farm is a website set up with the sole purpose of increasing the link popularity of other sites by increasing the number of incoming links to those sites and Link bait is any content or feature, within a website, designed specifically to gain attention or encourage others to link to the website.
Link farms are pages stuffed full with links that and connected with other link farms due link exchanges. i advice to stay away from linkfarms since they have very negative impact on your SERP