As a general note - the Stack Exchange model has been specifically written to allow such edits - this is not a problem or a bug - it is by design.
Now, to your questions:
how can i tell what they did and why?
Click on the
edited <relative time> ago link. You will see all revisions.
The revision should have a summary too - should tell you why.
do others see who has made edits?
what happens if I reverse those edits (and is there an automatic way to do that or do I have to manually re-edit?
You can rollback (link on the revision you wish to rollback to), but - make sure not to rollback changes that improve the post. Rollback wars are frowned upon.