Licensing companies work to get your music placed in TV shows, movies, trailers, commercials and video games. Independent licensing companies have been popping up left and right over the past 10 years. Before that (and still currently) all the synch licensing was done within publishing companies. All publishing companies have synch licensing divisions.
Licensing companies typically take a percentage of the up front synch license and master use license fee. Some take a percentage of your PRO backend royalties as well, others don’t.
Licensing companies typically only represent artists who are also the sole songwriters. Licensing companies are one stop shops for music supervisors. They want to make it easy as possible for the ad agency or TV show to use the song. Licensing companies can clear the songs immediately for the music supervisors.
So if you co-write with anyone, FIRST make sure they are NOT signed to a publishing company (if they are, it makes things very difficult and will almost certainly prevent a licensing company from working with you – or rather repping that song). And make sure you get in writing (email is fine), that you have full rights to the song to license without getting permission from your co-writers.