A strong name key file has a .snk extension and contains a unique public-private key pair. You use the strong name key file to digitally sign your assembly. Note that this type of file is not secure, as the private key in a .snk file can be easily compromised.
For added protection, Visual Studio can encrypt a strong name key file, which produces a file with the .pfx (Personal Information eXchange) extension. The .pfx file is more secure because whenever someone attempts to use the encrypted key, she will be prompted for the password.