I just wonder if moderators earn any money or earn a salary for being moderators - or do they do it just to help out on the site?
No they don't. It's an all volunteer effort.
Note that there are also Stack Exchange employees that do moderation work.
The benefits of being a moderator:
- A huge say in the direction of a site.
- Access to the site views.
- The ability to choose an organization of a list of a few to receive $100 once a year from Stack Exchange.
- More direct access to the Stack Exchange employees.
- That warm fuzzy feeling of making a difference.
- Access to the Teacher's Lounge, a chat room for moderators.
- Once in a blue moon, SE will hire one of the community moderators as "Community Managers".
- The ability to edit chat messages after 2 minutes.
As has been noted, SE staff also has access, who do get paid. But there are some 500+ (Maybe even more?) moderators, including myself, who don't receive a dime for their services.
Moderating SE is a volunteer effort. We don't get paid for it, and we are free to step down whenever we want to. It's not always a fun job, but it has its high points and you get a sense of satisfaction as you see a site progress, knowing that you were (and still are) an integral part of it.
Most of us also have a natural tendency to participate in every meta discussion that ever comes up (the fact that we get inbox pings for every new meta question helps). While our words on meta don't hold much more intrinsic weight (except in extreme cases where the community is very divisive over an issue), the community generally respects and follows our opinion on a matter, so we end up "leading" the community at time. Again, one gets a sense of satisfaction for this.