Compilation album

A compilation album is an album of songs, made by different recording artists, or by one group or artist at different times. Some compilations are greatest hits albums, which feature the biggest hit records of a given year, or a certain genre of music, or artists from the same era or location. 
Others are called "best-ofs" or anthology albums that are picked as favorites by the person who puts together the new album. Some compilations include only the biggest record chart hits, while others may include B-sides, hard-to-find songs, or songs that were not released earlier.