Domains are the endings on websites. Some are limited to certain types of registered organizations (ex. .edu is limited to chartered schools).
Educational institution. Sites using this domain name are schools ranging from kindergarten to higher education. Information from sites within this domain must be examined very carefully. If it is from a department or research center at a educational institution, it can generally be taken as credible. However, students' personal Web sites are not usually monitored by the school even though they are on the school's server and use the .edu domain.
Stands for network. There are all kinds of sites under this domain suffix. Information from these sites should be given careful scrutiny.
Only official U.S. government websites will have addresses that end in “.gov.” Information such as Census statistics, Congressional hearings, and Supreme Court rulings are included.
Commercial site. The information provided by commercial interests is generally going to shed a positive light on the product it promotes. While this information might not necessarily be false, you might be getting only part of the picture. Remember, there's a monetary incentive behind every commercial site in providing you with information, whether it is for good public relations or to sell you a product outright
If you use a non-profit organization's site, think about the organization's agenda and how it's likely to influence the way they present information. There are no restrictions on .ORG domain names. Anyone can register, and the domain can be used for any purpose.