Avoid using copyrighted music without permission
Most musicians, especially popular musicians, hold the copyright, or ownership rights, to their music. This means that the musician or creator maintains the right to decide if and when their music is used in a movie, TV show, or advertisement, etc. If someone wants to use a song in their movie, they must get permission from the musician before using the song (simply giving credit is not enough). This permission is often granted with the agreement to pay a licensing fee. The more popular the music, the more money will be expected to use the song.
Creative Commons and public domain music
Some music was recorded or published long enough in the past that the music is outside of copyright, or in the public domain. Other artists publish their music under a Creative Commons license, which generally means anyone is welcome to use their music, but many artists do ask to be given credit.
These sites offer music in the public domain or licensed under Creative Commons which can be used in video and media projects: