Web of Trust

Bob's Public Key + Trent's Private Key = Signature
Signature + Trent's Public Key = Bob's Public Key (Verified thru Trent)
        +-------------------+ 
        |    Certificate    |
        +-------------------+ 
        | Bob's Public Key  |
        | Trent's Signature |
        +-------------------+ 
    * Technically there is some other stuff in there too.