First read What is a certificate and what is it used for?
Certificates for people are called personal certificates.
Most people only need this kind. Depending on what you want to do, you
may need to get a KCA or a DOEGrids personal certificate. Most
Fermilab users are eligible for both.
You may possess only one valid personal
certificate at a time from a given CA. You may possess valid personal
certificates from multiple CAs. You can use your personal
certificate(s) in multiple applications.