Rinku first take a look at
this .
Any user has two keys associated with him one public key and another is private.
The terms themselves are self explanatory, the public key is made available to all those who wish to communicate confidential information to the user. And the user is in charge of his private key.
Now both the keys share a relation that anything encrypted with the public key can only be decrypted with the private key. In addition, although the keys share a mathematical relation, its nearly impossible to derive the private key using the public one.
More on public key cryptography
here . More on PKI
here.
Hope this helps