An optical sound track is a picture of a sound wave printed onto film stock next to the perforations. The standard method of analog optical recording is the creation of a variable area (V/A) track, which looks like a wavering line moving down the length of the film stock.
Before optical tracks were V/A they were variable density (V/D), but V/D is no longer used for new recordings. A V/D track looks like a bar code in varying degrees of opacity running down the length of the film stock.
Digital audio tracks (SDDS, and SR-D), and the DTS time code track are also optically recorded, and they appear alongside their analog cousin on the edges of the film stock.
Macro of 35mm film audio tracks, from left to right: Sony SDDS, Dolby® Digital, analog Optical, and DTS time code.
Image courtesy of Wikipedia Commons.
The four optical sound tracks are highlighted in this illustration.
This image shows a variable density sound track on the left hand side of the film frame.