Public key cryptography

Usually we close our doors with a key and then we can open that back only with the same key...

In Public key cryptography there are two keys - key1 and key2. If we close with key1, then we can open only with key2. And if we close with key2, we can open with only key1. Nice assymetry!

One key we dont disclose and keep it as secret - its Private key as name denotes, other key we can give to Public, and hence named Public key..

Some examples where i can use:
case1: say i want to send a mail to biju and i want only Biju to read that mail. So i can encrypt with Biju's public key and send. Only Biju will be able to decrypt the message (with his privatekey).

case2: in another case, i want to send a message, but i want Biju to be sure that its me who has send this message. Then i can encrypt with my private key. Biju will be able to properly decrypt that only(with my Public key).

So Biju and me are happy!