Here's a quick rundown of the most common aspect ratios:
1.33:1 - this was the OAR of silent films and early talkies, used until 1932. It is also the standard for most television programs, past and present, as 4:3.
1.37:1 - known as "Academy Ratio," it debuted in 1932, and was the standard for most motion pictures until 1953. If you're watching a film in this format on TV, nothing is cropped or lost.
1.66:1 - European widescreen standard.
1.78:1 - video widescreen standard.
1.85:1 - the standard format for most U.S. and U.K. theatrical films.
2.2:1 - a now-rare format, known as Todd-AO, because it was co-developed by Broadway producer Mike Todd, with the American Optical Company. Todd-AO is extremely high definition widescreen, and used only on a handful of films, mostly produced as "event" or "road show" entertainments, in which you often had to buy a ticket in advance, as if you were attending a Broadway show, and were given assigned seating. This was the first film format to utilize 70mm prints, followed by Panavision's Super Panavision 70 and Ultra Panavision 70, which are standards today. But Mike Todd pioneered the format.
Major films produced in 70mm Todd-AO include:
Oklahoma! (1955)
Around the World in 80 Days (1956)
South Pacific (1958)
The Alamo (1960)
Cleopatra (1963)
The Sound of Music (1965)
The Agony and the Ecstasy (1965)
The Bible (1966)
Dr. Dolittle (1967)
Hello, Dolly! (1969)
Patton (1970)
Todd-AO faded out in the early 1970s, but re-appeared in the early 1990s, and was most famously used that decade in Kenneth Branagh's Hamlet (1996), the last major production to date to use the original Todd-AO.
2.35:1 - or CinemaScope. Used extensively from 1953 to 1970. 35mm anamorphic.
2.39:1 - the current widescreen format, in use since 1970. Often rounded off to 2.40. As Mrs. Pepper said, the black bars on the top and bottom of the screen don't actually exist. The picture is wider than, say a 27-inch TV screen. Despite the smaller size, you're getting approximately everything the director wanted you to get, without having to crop or pan-and-scan the image.