All questions have the user's name at the bottom right. But some don't have the avatar and have the mysterious "community wiki" over the user's name. What exactly does that mean?
Here's an example: Not-So-Well-Known features of Ubuntu
This is the canonical source of what community wiki is about:
According to the StackOverflow FAQ:
Community Wiki posts work by transferring ownership of the post from the original author to the community. They make the post easier to edit and maintain by a wider group of users, but they do not contribute to any user's reputation.
CW posts have lower thresholds for editing (usually around 100), among other things.
See the relevant StackOverflow meta question for more details.