v2s, v3s (and so on) are basically revisions of the original file. If something went wrong with the original file, or the group decided they could improve some aspect of it (not necessarily something wrong with it) and they deem it important enough, they will release a second version of the file. There are many different reasons why a group would release a v2. I've seen v2s released for translation errors, encoding errors, and just for downright no noticeable reason.

The rule of thumb for v2s is this:
-If you already have the v1 and see nothing wrong with it, keep it.
-If you haven't gotten the episode yet, get the v2.

Hope that helps!
P.S: Sorry for revving this old topic, but I found joker-kun's post a little too vague and I thought I could add something to it.

-Karasuhebi