What is in the other category on my iPhone 4S capacity?

3.5 gb of my phones capacity is being taken up by this others category and now my phone has no space. What is in it and how do I clear it?

It is the system files. You can't delete it without rendering your phone useless. If I were you, delete any songs, apps, or videos you haven't used the past 2 months. That's what I do at least.

Yes, check the file list in your iPhones, there must be some useless files, such as games, videos, musics. You can delect them to earn more capacity.