Indeed, anyone can propose edits to a question or answer. For those with insufficient privileges to edit outright, these proposed changes go into a queue that is visible to users with 10,000 reputation (or more) and moderators. These users can inspect the change and approve it, reject it, or delete the entire post. If approved, the person suggesting the edits gets 2 reputation points. You can earn up to 1,000 reputation points through suggesting edits; of course once you reach 2,000 points you can edit without prior approval. (Note, of course, that all edits are recorded and can be rolled back if necessary.)
When you edit someone else's question or answer, keep these things in mind:
- Fix typographical errors
- Correct spelling mistakes
- Fix broken links
- Remove e-mail addresses
- Improve formatting
- Take out greetings ("Hi!") and sign-offs ("Thanks in advance -- Username")
- although, if this is the only reason you are editing the post, it is probably better to leave it alone
However, and more importantly...
- Change the meaning or content of someone's post
- Add your own commentary
- Delete content
- Add information
In other words, your edits should change the readability of the post, not the content. Otherwise, leave a comment.