Musical films have been around for many years, the films originated from theatre productions, from 1926 until 1929 musical films were just being introduced to the media industry, in 1929 the films got their break with 60 musical films being made in that year alone. In fairness none of these films are well known today and have not been ones that are memorable but the fact all those films were made in one year alone shows the jump in popularity for these films.
http://en.wikipedia.org/wiki/List_of_musical_films_by_year (list can be found at this link)
Musical films are often seen as your all singing all dancing films such as west side story one of the most famous musicals to ever be made but in recent years they have widened as films such as High School Musical which is a film for younger people created by Disney, which went onto to have 2 sequel films.
Here is the west side story trailer you can see from the image quality how old the film is.
Here is a High school musical trailer
You can see from the quality of the two films which one is the most recent, even though the films look very different they will all encoperate the majority of the same features.
Features of a Musical :
Singing
Dancing
Normal conversations sung not spoken
Often a relationship between two people is where most of the singing happens (passion / anger)
Songs move the film forward and widen the plot
Actors looking directly into camera if a solo
Lots of background characters.
