In symmetric cryptography, a shared secret is a secret key shared between the two parties.
It's also known as symmetric encryption key (ie in contrario to asymmetric)
Since the same key is used both to generate the signature and to validate it, care must be taken to ensure that the key is not compromised.
It's categorized as a something you have authentication.
The public key cryptographic scheme (asymmetric) is often used to exchange an on-the-fly symmetric key, which will only be used for the current session because it's much more performance efficient.
The most known example is SSL. At the start of a SSL connection (known as SSL handshake or negotiation), the two parties are using an asymmetric public key scheme to create and exchange a symmetric key that will be used to communicate for the time of the connection.