what are the requirements/benefits of an employment visa?

Answer 1

Your question is not very specific. For example, what country are you asking about?

Here is some basic information about US employment-based immigrant and nonimmigrant visas:
http://www.travel.state.gov/visa/immigrants/types/types_1323.html
http://www.travel.state.gov/visa/temp/types/types_1271.html