This is a very deep subject - so we will skim the top and show you what you need.
Subtitles are similar to "Captions" - they display text at the bottom of the screen that follows the words being spoken on the video. The most common application is for foreign films - the subtitles let you see the works in English. Subtitles are a separate stream of data that is combined with the Video stream. They can have just one language or they can come in multiple languages - i.e. multiple streams.
Subtitle FileTypes
There are many formats of subtitle files, each is designated by the file extension. The most common are SUB/IDX,.SRT, SUP, and SSA. Ther is also SMI, TXT, and about 20 others.
SMI - Microsoft has standardized SAMI 1.0 (see http://msdn2.microsoft.com/en-us/library/ms971327.aspx ), which have the extension "smi". You can test and play any video file with smi file on this test page: http://users.pandora.be/vlad/subsync/mplayer.htm
SSA (SubStation Alpha) - very popular because a plugin is available for VirtualDub which will combine SSA subtitles with AVI movie files, creating a new movie file with "hardcoded" subs
If you have a "sub" file then you also need the corresponding "idx" file - the sub file in compressed text for the words, and the idx file is a text file containing the timings and synchronization of the words (i.e. it tells the media player "when" to display each set of words). The other file types are all singular - just one file contains all the necessary data.
When downloading movies on the Web where you have the movie file with no subtitles and then a separate subtitles file - there are two ways to view them on your PC - you will need to test and use trial and error to see what works best for you and your video card. Subtitles are very very picky and often do not work unless in a particular format:
Downloaded Movies (from Bit Torrents)
Creating Subtitle Files from DVD's - for Uploading to Bit Torrents
There are numeropus ways to do this - all of them a hassle. You can use "SubRip" to pull the subtitles off the VOB files from the DVD, then convert the main movie to an AVI and test the synchronization using Subtitle Workshop and adjust if necessary ( usually it is necessary).
Copy the VirtualDub plugin "subtitler.vdf" into the PLUGINS folder
under the VirtualDub program directory - the filter will then automatically load
when
the main program starts. If the PLUGINS folder does not exists, create it.
Synchronize the Subtitles if Necessary - start up Subtitle Workshop or use SubSynch. Load the movie into the preview pane, then load the subtitle file - preferably in SRT format. Play a few sements and check the synchronization of the audioble spoken words with the subtitles flashing on the bottom of the video.
What is the Best Timing to Use ?? The words of a sentence should pop up onto the screen either "exactly" when the speaker says the first word, or after a brief delay (about 1/2 sec into the spoken sentence). Do NOT have the subtitles pop up BEFORE the first word of the sentence is spoken - that just "feels uncomfortable" as you watch the movie.
Convert your Subtitles File into SSA Format - the VirtualDub subtitle filter works with ssa subtitles ONLY. So if you have the more common sub or srt or some other format, you will need to convert it to ssa.
Naming Convention - although not always necessary, to create a good habit - name the subtitle file the same as the movie. For example, if the movie file is "Miami Vice 2006.avi" then name the subtitle file "Miami Vice 2006.ssa"
Customize the Subtitles (color, font, placement) - even if your subtitles are already in SSA (SubStation Alpha) format, this step is recommended to customize the subtitles aspect. We'll use DivXLand Media Subtitler to convert the subtitles to SSA and/or edit their font aspect and position. Open the subtitles with DivXLand Media Subtitler from the File menu.
Then go to Settings > Format Settings. In the popup dialog select the SubStation Alpha format.

Configure the format with a clear font such as Trebuchet MS and a font size of 36pt. Here you can also change the subtitles alignment and margins if desired.
After that you can save the subtitles from File > Save As. The following dialog will popup:
Make sure to select the
SubStation Alpha [SSA] format and click OK to save the subtitles.
The next step is converting
the .srt files into something VirtualDub can
read, in this case, an .ssa file.
The final step is selecting
the compression for our .avi.
To finish up simply go to
File and select Save As .avi or
simply press F7 for convinience. Save it where
you want and make sure the Save as type
is set to audio-video interleave (avi).