A single is a song that is released separately from an album, but usually also appears on an album. Often, these are the most popular songs from albums that are released separately for promotional uses such as commercial radio airplay. Singles usually are released with a music video.
Demi Lovato has a number of 11 official Singles with music videos that are released on studio albums to date. She also has a number of other singles released with no music video such as the song This Is Me from the Walt Disney's Camp Rock (soundtrack).