I have noticed that I can edit other people's questions on Worldbuilding StackExchange. Does this mean I am one of the Moderators of Worldbuilding StackExchange?
...in the sense that everyone who contributes to Worldbuilding is a moderator:
We designed the Stack Exchange network engine to be mostly self-regulating, in that we amortize the overall moderation cost of the system across thousands of teeny-tiny slices of effort contributed by regular, everyday users.
As you contribute here, you earn more and more extensive moderation privileges, giving you considerable influence over the direction of the site. You're trusted to collaborate with others in establishing rules and norms, and to make sure that they're communicated to the folks just starting out.
There are also elected moderators who have even more extensive privileges and are expected to perform roughly the same duties (though perhaps more often and more extensively). When folks talk about "Moderators", they're often referring to this latter group, although they can be referring to either.
No, it is a privilege granted based on your reputation. One level lets you edit to make the posts better, but all of your changes are reviewed by other users. A second level of edit authority is granted at a higher reputation. When we get our new site design all of the privileges are moving up to 'full' site values.