A person who responds to anonymous sex/casual hookup ads posted on-line. Can be used in a general sense to describe people of either sex seeking either gay or straight sex, but is more commonly used to refer to older gay men responding to ads posted by younger men, whether or not they expect some kind of compensation.
A person who posts such an ad is said to be "trawling for squid."
Ted needed some extra cash and also was hoping to meet someone who could maybe point him to a better job, so he posted a few ads on Craigslist, trawling for squid. But with his lousy luck, he got busted by an undercover statie when he went to go blow some dude at a highway reststop.