There are two types of government services:

Competitive service: These are the most common Federal jobs for which applicants must meet all the qualification requirements to be invited for interviews. The competitive service is under the juridiction of the Office of Personnel Management, which assures that all candidates will be treated fairly and equally. 

Excepted service: These are the jobs for which agencies set their own qualification requirements (for example the CIA and FBI). Another important thing to know is that these jobs are subject to Veteran's preference.

Whatever the type of service you will be applying to, make sure to follow all the requirements listed in the vacancy announcement and have an appropriatly formatted Federal resume.