Whenever you release a recording of a song that someone else wrote in a video format, even if it's just a small portion of the song, you need a synchronization license. Synchronization licenses are most commonly used for YouTube videos, cover song videos, wedding videos, and commercial and corporate videos. For example, if you release a YouTube video of your band playing a Rolling Stones song, even if you use only a portion of the song, you need a synchronization license. If you release a DVD of yourself playing a Beach Boys song or singing Mariah Carey lyrics, you need a synchronization license.
A synchronization license is required no matter how small a portion of the song you use. There are some exceptions where a synchronization license is not required: You don't need a synchronization license for songs that you wrote yourself or songs that are in the public domain.
Note that synchronization licenses are for video products (DVDs, YouTube videos, other web videos, and slideshows). If you are creating an audio-only product, such as CDs or vinyl records, you need a mechanical license instead. Mechanical is for audio-only, synchronization is for video.
If you use an original sound recording belonging to someone else (for example, an actual Beatles recording featuring John Lennon, Paul McCartney, Ringo Starr, and George Harrison) you will need a synchronization license to pay the composer for the right to use the composition (song), and also a master license to pay the artists for the right to use the recording. This is true even if you are sampling only a very small portion of any existing copyrighted audio recording.
If you display lyrics or music notes in your video you will also need a print license to pay the composer for the right to display the composition (song).