What Is the Difference between a Nonce and an Extranonce?

The nonce is a 32-bit field in the standard block header that miners iterate on to find a valid hash. The extranonce is a variable-length field contained within the Coinbase transaction, which itself is part of the Merkle Root.

When the 32-bit nonce is exhausted, the miner changes the extranonce. This modification creates a new Merkle Root, which in turn creates a new block header and allows the 32-bit nonce search to begin anew.