Adrian has written a wonderful overview of how Public Key Infrastructure words. If you don't know your certificate from your asymmetric widget then this is an excellent place to find out how it all hangs together.
Adrian provided the following introduction:
"These documents were written at the request of colleagues at a recent client; technically savvy guys but unfamiliar with PKI. Hence the rather demotic tone. I have been concerned to clarify aspects which caused me much confusion when I was learning the topic, in particular to keep very distinct the technology - asymmetric cryptography - which makes the whole thing possible, and the administrative process - PKI and all the rest - which controls it."