A dozen is a unit of measurement. It means twelve (12) items of something. The term goes back to duodecim, which means 12 in Latin. Humans might have started to count on a base 12 because there are approximately 12 cycles of the moon in one cycle of the sun. In other words, a solar year has 12 months. The first to have used the unit were probably the Mesopotamians.
The dozen is convenient because its multipliers and multiples are convenient: 12 = 3 × 2 × 2, and 360 = 30 × 12. The French word douzaine means "about twelve" and is part of a family of words with similar meanings, e.g. vingtaine – "about 20" centaine – "about 100" etc. A baker's dozen, also known as a long dozen, is thirteen.