While we inherit most of our editing conventions from the Stack Exchange (SE) network. I feel it is important to rephrase them here, and possible specify some extra guidelines specific to our site.
Almost anyone can do suggested edits, but these will need to be peer reviewed first. The guidelines you receive once you have editing privileges (1000 rep) mention the following (emphasis mine):
When should I edit posts?
Any time you feel you can make the post better, and are inclined to do so. Editing is encouraged!
Some common reasons to edit are:
to fix grammatical or spelling mistakes to clarify the meaning of a post without changing it to correct minor mistakes or add addendums / updates as the post ages to add related resources or hyperlinks Try to make the post substantively better when you edit, not just change a single character. Tiny, trivial edits are discouraged.
What happens when I edit a post?
The post will be updated to show the latest editor, as well as the original author. All edits are saved and tracked in a revision history with attribution to each editor.
Editing a post also bumps the question to the top of the homepage. Too many edits can also force a post into community wiki mode. Make your edits count.
Choosing when to edit isn't always straightforward, so there are plenty of existing discussions on the parent meta site. When your question isn't specific to this site, you can search for/ask it there.
I'd like to point out one important resource on editing: In Defense of Editing, written by Jeff Atwood.
If you are going to edit a post, make sure you’re substantively improving it. Avoid making isolated, trivial edits, as they are the source of much friction.
Be diplomatic in your edit-related comments. Explain that the spirit of SO is collaborative editing, and you’re only trying to make substantive improvements (see rule #1). Above all, be nice. If there’s any resistance — even unwarranted and unjustifiable resistance — just let go and move on.
These are some of the guidelines which have already been established and which we should follow on this site as well. Feel free to leave any additional site specific issues/examples/remarks as an answer.