[new-user] question

[michael crane]

hello,
I'm trying to understand the principals and benefits of using pgp/gpg
I think I understand that I send the part of my key that is public to
somebody and they use that key to encrypt a message which only I can
decypher.
So what if somebody uses my public key to send me a message purporting
to come from somebody else ?
what is the mechanism to ensure it came from who I think it did ?

regards
mick