A song is a relatively short musical composition for the human voice (possibly accompanied by other musical instruments), which features words (lyrics). It is typically for a solo singer, though may also be a duet, trio, or for more voices (works with more than one voice to a part, however, are considered choral).
In music, a single is a short record, usually featuring one or two tracks as A-side, often accompanied by several B-sides, usually remixes or other songs. Most singles have only one A-side and are named after this song, but some may have a double A-side (a famous example being Strawberry Fields Forever/Penny Lane by the Beatles), where two tracks are given equal billing in the title of the single.
A music video (also promo) is a short film or video that accompanies a complete piece of music, most commonly a song. Modern music videos are primarily made and used as a marketing device intended to promote the sale of music recordings.
A film soundtrack is the music that is from or inspired by a feature film. Soundtracks themselves are not limited to film. One may find soundtracks to television shows, ranging from ER to the anime Cowboy Bebop, and video games such as the Final Fantasy series.
Eminem vs Everlast