Although both are female bovines, one difference between them is that a heifer is a young female under the age of 3 years while a cow is an older female that has given birth to a calf. The term "heifer" refers to a young cow that is weaned, is not over 3 years and has not had a calf. Sometimes, the term "cow" may be confusing because it is often used to refer to all female bovines.
However, there is a clear distinction between a heifer and a cow. A heifer is called a cow once she has a calf. Another way to differentiate between a heifer and a cow is by observing the udders. A cow will have developed udders because it has given birth and produces milk for its calves.