Certificate Authorities
Certificates are essential for using asymmetric public-private keys
RSA set up VeriSign company which offers web certificates for individuals, servers and software publishers
There are many other authorities
See http://digitalid.verisign.com/info_ctr.htm for a good description of certificates both in general and for Verisign services (http://www.verisign.com)
Individual certificates are for use in Web browsers and for secure web mail(S/MIME) such as that offered by Netscape
There is no agreed format for certificates but X.509 v3 certificate is common (PEM extends this)