What is difference between baldness and alopecia?

Baldness involves the state of lacking hair, most often on the head.  The most common kind of baldness is what is called "male pattern baldness" in which the crown of the head and top lose hair.  This is medically known as androgenic alopecia.  This type of alopecia can also occur in women, but they seldom lose all their hair; it just gets thinner all over and the hairline does not recede.  This type of alopecia is hereditary and accounts for 99% of hair loss.

There are also other types of alopecia.  For instance, alopecia areata is manifested sometimes in patches on the head, often with one side more affected than the other.  Hair loss tends to be rather rapid.  Other alopecias occur after very high fevers or severe illness. high doses of Vitamin A, or retinols.  This hair loss is often temporary, however.

So, baldness is the state of lacking hair, while alopecia is a medical condition which causes various types of baldness.