They are apps or functions built into apps that scrape the databases of certain sites to pull and store info about your file. For movies you'll get all the imdb info, posters etc and TV shows will do the same with sites like TheTVDB plus grab all the different episode synopsis.
There are a few standalone ones, only one i used to use was Media Companion but now i just let XBMC do all that itself as it's good enough these days that you don't need a separate scraper dumping files in your folders.