.com - commercial sites
.edu - schools, colleges, universities
.gov - state and federal government sites
.org - generally nonprofit/NGOs
Whenever you are considering using a website as a source, it is very important that you review the contents in detail. An easy way to make a determination is to use Jim Kapoun's Five Criteria for Evaluating Web Pages:
There are many other resources to evaluate web pages. One of the more interesting examples is CSU Chico's CRAAP Test.