Subtitles can be categoried by the following things: script based or pictures; embedded into multimedia files like avi, mkv and so on or in one or more seperate files
The simple script based Subtitles are formats only give a time position and the Text which should be shown. More advanced formats know also styling(color, font, font size ans so on), and positioning(visual, like centered bottom).
The best know picture based Subtitle format is the DVD subpicture format. The picture based subtitles are good for none Computer based environments like DVD Player, TV and Videoconferencing.