The main difference between 401 and 403 errors lies in their meanings. A 401 error, "Unauthorized," means that the request lacks valid authentication credentials for the target resource. On the other hand, a 403 error, "Forbidden," means the server understood the request but refused to authorize it. While both involve a lack of access, the 401 error implies that the user can authenticate (i.e., log in) to gain access, whereas the 403 error means access is completely forbidden.