Hello Heather,
Downloading any type of music, even music in user-made videos, is considered illegal unless you personally pay for it and the company you pay is legally able to distribute it. Some legal companies do have specials, however, that give you a certain amount of free music with intent to 'rope you in' or simply as bonuses, etc. that may be legal as well. The popular free download site Limewire was an example of an illegal means of obtaining music (it was shut down by feds, and some of it's "customers" were then sought out for illegally distributing music/obtaining music illegally). Hope this helps. You might find more info by typing questions into google or bing that will give lists of what others say about it:
http://www.bing.com/search?q=what+is...15&sp=2&sk=AS1