In films like Step Up, Wikipedia shows some songs as featured songs and some songs as non-featured. Even only featured songs appear on their album. So what are those non-featured songs and what are the rules regarding the usage of non-featured songs? I mean, are movie makers free to use any song as non-featured song or are there any rules regarding it? I mean doesn't they have to pay for them?
Anything beyond 'fair use' of a copyrighted piece of work is illegal without getting permission or a license to use it.
Description of US copyright 'rules' for fair use can be seen at copyright.gov. Clearly interpretation of fair use is complex but basically the main aspects are:
Now using the music in a movie arguably might boost the sales of the original song, so an artist or the owner of the work might give permission and not demand a fee. Clearly anything featured in a soundtrack album has potential to negatively impact sales of the original, someone might choose to buy the soundtrack rather than the original to obtain a copy of that song - so a license fee would normally be involved.
However as the document from copyright.gov states - its safest to obtain permission from the owner of the work. If they demand a license fee, then its up to you to decide whether its fair use or not risk using it anyway or to decide to pay the fee, or not use the music.
The RIAA have even been known to send cease-and-desist letters to YouTube users who have music in the background of their amateur non commercial videos so this is a hotly contested area.