Only official Government sites can use that domain. It's so users know it is genuine.
top-level domains
.com , .org , .edu , .gov
Yes
.Gov tld domains cannot be purchased by ordinary organizations or person - only people from the government are allowed to operate these domains, I'm not sure but i think .gov has more importance in search results over .com such as Google.
All of these are domains name: .edu, .gov, .com and .org.
.gov, .org, .sch.uk
government
government
To get a backlink from .edu and .gov domains is a difficult task, the easiest way i recommend that you can gain such backlinks is by taking part in projects run by such .edu domains. By projects i mean most .edu domains are owned by Universitys if you some how contribute to projects that they are undertaking you will be able to place a link to your website from your profile page.
commercial websites base their information around selling their product
SBA in SBA.gov stands for Small Business Administration. The gov stands for government.
A Government Owned Vehicle (GOV), in contrast to a Personal Owned Vehicle (POV).