I usually present this information in the continuing education class I teach for grant writing because it allows agencies to determine if they are calculating salary requirments correctly for staff positions, but its just as useful in determing on an individual basis if you are getting paid appropriately for your job and developing an arguement for pay raises based on national, regional and local averages in your field.
A Salary Survey is essentially a database of common job titles and job duties and what everyone is paying people to do that particular work. Surveys are used by companies and non-profit agenies to track disparity among gender and ethnicity (racism and sexism) and also skill value (what skills sets are so needed that you have to compete to get the best employees).
Employees and consultants and contractors use survey to issue salary range requirements or bid on contract work according to whatever else sees as a reasonable fee for that person's background. It is also used to argue why you should get more that other people, because you have more skills than most people that do particular types of work.
Here are a few sites that issue salary estimates"
There are endless number of companies, agencies and sites that produce and disseminate this information. Some have limited access for free users. But if you are accepting a position that is good money, it's worth it to pay a $20 fee or so to take a peak at what other employers are paying for someone like you.