The vellus hair is the type of hair that is characteristically very fine, soft, and non-pigmented. It is the type of hair that replaces the lanugo in fetuses and newborns. Lanugo is the first type of hair that is produced (in utero) by the hair follicle. It covers the body of the developing fetus and is believed to aid in holding vernix caseosa on the skin of the fetus. Lanugo is typically shed before birth and is replaced by the vellus hair at about 33 to 36 weeks of gestational age. However, there are also several instances when the lanugo hair is not shed at birth and only replaced by vellus after a few weeks, post-natal. Compared with lanugo hair, the vellus hair is less visible. Typically, the vellus strand is <30 μm in diameter and < 2 mm in length.1 The vellus hair is the type of hair that covers most of the body at childhood to adulthood. At puberty, vellus is replaced by terminal hair (specifically called androgenic hair) in certain parts of the body.2
The function of vellus hair includes body temperature regulation (through thermal insulation and cooling).
1 Peytavi, U. (2008). Hair growth and disorders. Berlin: Springer. p.9
2 Hiort, O. "Androgens and Puberty". Best Practice & Research Clinical Endocrinology & Metabolism 16(1): 31–41.