An actor is the individual who acts in mediums like theatre, film, television or radio. By definition the word means an individual who interprets. And a singer is an individual who sings i.e. performs vocal music.
Though by definition these do not coincide, but they are also not mutually exclusive. Acting, singing, dancing etc are talents multiple of which may happen to exist in any one individual.
There are people who enter the entertainment business as actors and then turn into singers. The transition may also be reversed i.e. a singer turning into an actor.The whole transition depends only on existence of talent and skill in the individual.
In the recent years there has emerged a pattern where a lot of actors/ actresses have made a transition into singing as a parallel or alternate career choice. This pattern has been more evident in Hollywood.
There are a lot of actresses who have tested the waters are singers. Scarlett Johansson, Gwyneth Paltrow, Leighton Meester and Nicole Kidman are amongst those females who are both actor and singers.
The fact is that the actresses have not just limited themselves to acting and singing. Most actresses nowadays indulge in multiple segments of the entertainment business.
Demi Lovato for example is an actress turned singer, who is also a musician- songwriter, a dancer, a spokesperson and a television personality.