For many movies (those that make good use of it), the sound track is just as important as the pictures and the story. It helps to set the mood of the film, and just isn't the same without it. Sound effects are usually added to add to the realism (including simple things like footsteps, and the sounds objects make as you pick them up, which would make a scene look strange if it were totally silent). And most movies usually have a music track, too. For many of them (e.g. Alfred Hitchcock's famous films, Star Wars, Star Trek, etc.), it was a major part of the film, with the music is specially written to suit the film, rather than just fitting in some music that already existed (although 2001 and did that rather well).
There are various ways that movies can have sound tracks. Originally, they had monaural (mono) tracks. A single sound channel, where all the sound came from it (rather obvious, really; as there's no alternative). With many cinemas placing the speakers behind the screen, so the sound emanates from the same area as the picture. Later on, came stereo, multi-channel, surround sound, and some other tricks (e.g. sub-sonics that you feel, rather than hear).
Stereo sound typically means two channels, usually used for left and right sound tracks. The idea being that anything that you saw on the left side of the picture would have most of it's sound coming from the left sound channels, and vice versa. And anything that was central to the picture would be heard from both the left and right channels, in an equal amount. Other sounds (e.g. music, and background ambient noise), may be placed off-centre, but still present in both left and right (i.e. not in equal amounts), so that it has a more realistic three-dimensional quality to it.
Cinemas often included a centre/front speaker, so that any sound that was equally present in the left and right speakers would be fed to the centre speaker, to help ensure that the sound did seem to come mostly from the centre. This helps the central sound actually sound more central, for anyone who's seated non-centrally, and rather close to one of the left or right speakers. This front speaker was often fed with a matrix of the left and right signals, though some films have a specifically recorded centre channel.
Cinemas also often have a rear speaker, either fed with a specific rear-channel sound track, or derived from matrixing the left and right signals together (to pick out the “difference” between the left and right tracks; the left signal minus the right signal), and often fed through a delay system (to give a bit of an echo). The rear channel adds an extra ambience to things, so you're surrounded by sound. Sometimes this would just be background noises, so you'd feel like you were sitting in a scene, rather than looking at one through a window. Sometimes effects are used so that if a plane flies overhead, you hear the sound travel from front to back (or vice versa, depending on the scene).
Then came multi-channel sound, where a variety of sound tracks feed individual speakers spread around the auditorium. So that sounds could be made to travel around you, giving the illusion of motion (again, imagine the example of a plane flying overhead, and being able to hear the direction that it's flying across). Walt Disney's “Fantasia” was the first film to do this, with a six-channel sound system that they called “Fantasound”. It required the cinemas to be specially kitted out with a new sound system. Now many movies have multi-channel sound tracks (typically front-left, front-centre, front-right, rear-left, rear-right, and a non-directional sub-woofer for the really deep sounds; known as 5.1 for the 5 main sound channels plus the sub-woofer).
Some movies and cinemas used the sub-sonic sounds (pitches that are so deep that you often feel them more than hear them) to physically shake the floor or the seats, to give special effects to some scenes (imagine watching a roller coaster ride, where you can also feel the rattles as it goes around the tracks, or film with an explosion where you can feel the shockwaves).
To be continued… (THX, DTS, Dolby, etc.)