You can find more information about kashrut at the websites of major kosher certification organizations.

The Orthodox Union , which is responsible for "OU" certification, has some excellent information on its website, including a kosher primer, an explanation of their kosher policy, a philosophical discussion about "thinking kosher" and a questions and answers section. (Please note: the "Judaism 101" listed on some of their pages is not this website and has no connection with this website).

The Star-K Kosher Certification organization also has an excellent website.  The wonderful thing about Star-K is, they give you an incredible amount of detail about the research that they put into determining whether a product is kosher. They tell you what products may be used without kosher certification, and they explain why such products can or cannot be used without kosher certification, giving complete detail about the research that went into making their determination.  It also has articles about kashering appliances, and other useful information.

KosherQuest has a searchable database of kosher products as well as an extensive list of reliable kosher symbols and other interesting things.