Fake Twitter Followers

Can someone explain to me please what the deal is with the spam-like followers who occasionally decide to pursue me on Twitter? You know what I mean...they're "people" with seemingly real names and a picture who randomly start to follow you, but if you click back to their profile all you see in their updates are url's and references to porn and marketing stuff and blahhh. Here's one, for example:


Are these 'bots generating these accounts and haunting people, or an individual effort or what?