You can learn a lot about a website from its address.
.com means that the site is commercial, meaning that it was bought and is owned by an individual or company. These are the most common kind of website and you must be careful when using them.
.gov means that it is a government website. These often have valuable and reliable information on specific topics.
.edu means that it is an educational website, normally owned by a school. These can be good, but keep in mind that you may be looking at a grade school or student's webpage hosted by their college.
.org originally were owned by non-profict organizations, but now anyone can own a .org. Treat them like you would a .com and be very careful. For example: anti-dolphin.org is not a reliable website.
There are others (.net, .us, .mil, etc.) too, but these are the major ones.
Ask yourself: Is this source CRAAP?
Check the source for:
Currency: When was the source made and was it updated recently?
Relevance: Is it related to your topic and what is its intended audience?
Authority: Who are the authors and what are their qualifications? (If you can't find the name of the author or a sponsoring organization, be very careful with that website/source.)
Accuracy: Where does the information on the page come from, is supporting evidence given, and has the information been reviewed by others.
Purpose: What is the purpose of the source: to entertain, persuade, or inform? Is the page impartial or biased?