How do you find the domain and range of an equation?

Since this is not my area of expertise, I searched Google under the key words "domain range equation" to get these possible sources:

http://education.yahoo.com/homework_help/math_help/problem_list?id=miniprealggt_10_1
http://id.mind.net/~zona/mmts/functionInstitute/definitionOfFunction/definitionOfFunction.html
http://www.analyzemath.com/

I hope this helps. Thanks for asking.