What IS Guitar Hero custom?  

I've only played Guitar Hero at friends' houses and don;t own it myslef, so please bear with me. >w<
I've seen a lot of videos on Youtube that say "guitar hero custom" and show gameplay of a song that's not on the original game, and the guitar parts go exactly with th song.
So how do you put the songs you want on the game? Do you have to hook it up to your computer or add some software to the game? and do you have to make the guitar parts yourself, or does the game do it automatically?