Notaries certify the identity of their friends.
The traditional X.509 approach to PKI trust uses a hierarchy of trusted Certifying Authorities, chained off one another, with all trust consolidated in the Root Authority. He replaced the centralized PKI system with a network of trust statements made by individuals about one another. In theory, over time, that network would grow to encompass all PGP users around the world. Before deciding if you trust a PGP key, you can view the assurances people have made about that key and its owner, and the assurances other people have made about those people, and so on until you are comfortable with the level of assurance provided by the key. This system allows you to set complex trust parameters and policies based on the people who have made assurances about any given key.