SubStation Alpha (SSA), or the more recent Advanced SubStation Subtitles (ASS), are different versions of the same subtitle format. It may have the file extension .ssa or .ass, respectively. SSA is used for versions <=4, ASS is used for versions 4+.
The original site, which is down, can be found as a June 2003 snapshot here from the Internet Archive for more information.
An example document looks like this:
[Script Info] Title: Original Script: Original Translation: Original Editing: Original Timing: Original Script Checking: Synch Point: Script Updated By: Update Details: ScriptType: v4.00 Collisions: Normal PlayResY: 864 PlayDepth: 0 Timer: 100.0000 [V4 Styles] (Format: "Style: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, TertiaryColour, BackColour, Bold, Italic, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, AlphaLevel, Encoding") Style: Default,Arial,20,65535,65535,65535,-2147483640,-1,0,1,3,0,2,30,30,30,0,0 [Events] (Format: "Dialogue: Marked, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text") Dialogue: Marked=0,0:02:32.85,0:02:34.33,*Default,1,0000,0000,0000,,Oh no! The eggs are hatching!