What is the difference between ssl public key and pgp public key....
Two big differences are packaging and intended use. In both cases, the 'key' is just a big random number. However, surround data (i.e., domain, email address, signatures) differs between the two. Additionally in most cases an ssl public key* is intended to help secure an http or other network connection while a pgp public key* is intended to allow encryption of files and/or emails. Another difference is the almost all ssl key pairs are RSA keys, while many pgp key pairs are dh/dsa. (This describes how the public key and private key interrelate.)
Why they call ssl public key as as certificate why pgp public key was not called as certificate.
In both cases, the public and private keys are numbers. However, a certificate is not a key. Rather it is the assertion by someone that the key in question belongs to a certain entity. In pgp terms, a certificate is a signature. The keys for pgp typically have many signatures. In the ssl (or X.509) realm this would be treated as a number of independent certificates.
If you want more in depth answers, a good place to start would be the "Intro to Cryptography" document written by Phil Zimmermann and distributed with just about every version of PGP.
*In both cases the public key alone is of little value without one end of the transaction/communication having the associated private key.