Editing your posts
Some sites place a restriction on when you may edit your posts, and when it's indicated that you have done so.
A ninja-edit is when you 'slip in and out' to make an edit shortly after initially hitting the Reply or Create Topic button, and there is no indication that you've done so. Unless someone notices the change in the short window you have available to do this. At the time of writing, this is 1 minute after initially posting.
Regular edits are ones that people can see that you (or a member of staff, or @system) has made, and this is indicated by a diagonal pen near the time-stamp of your post at the top right. A recent edit is indicated by a red pen, a last edit some time ago is grey. At the time of writing, you may edit your own posts up to a 24 hours after initially posting.
Note that @system regularly caches locally any images you've linked to in your post, and the re-writing of the img src in your post shows up as an edit.
Viewing edits history
You may make your edit histories visible to other users (TL4 and under, admins and mods can see it regardless) by checking the following box in your preferences:
Note that this will merely prevent people seeing previous revisions. It will not hide the fact that there's been a non-ninja edit to your post.
If you inadvertently include some sensitive information, it may still appear in your edit history (subject to the previous item,) An admin or moderator can redact the previous revision from the viewable edit history so that only an admin or moderator can see it. Flag the post with a message to moderators to request the redaction.