public-key/two-key/asymmetric cryptography involves the use of two keys: 
a public-key, which may be known by anybody, and can be used to encrypt messages, and verify signatures 
a related private-key, known only to the recipient, used to decrypt messages, and sign (create) signatures
infeasible to determine private key from public
is asymmetric because
those who encrypt messages or verify signatures cannot decrypt messages or create signatures