NSF-certified cookware is cookware that has been certified as safe for consumer use. The NSF is the National Safety Foundation and it works in conjunction with the World Health Organization to test and rate cookware.
The NSF is responsible for testing all kinds of kitchen cookware and tools. However, it is important to note that having cookware NSF certified is strictly voluntary and cookware manufacturers do not have to subject their cookware to tests. However, because a piece of cookware does not carry the NSF seal doesn't mean it is not safe. The NSF seal is generally more important for those working in a professional kitchen than those who cook at home.