A SAL is a stitch-a-long. There are several different kinds. Stitchers may gather in person and work on the same project together or members of a board or forum may work at the same time and post photos documenting their project.
Another type of SAL is a Mystery SAL. A designer will post parts of a pattern and stitchers will complete it without knowing exactly what they are stitching until the pattern begins to be revealed.
Some stitchers have SAL weekends where stitchers each work on a different project. These also may be in person or online.
COMMENT: I’ve been known to do a project just to join the SAL. I especially like seeing how needleworkers change design colors to make it more vibrant.