Zcash (ZEC) is a cryptocurrency token similar to bitcoin in that it can be used in peer-to-peer transactions or bought and sold on exchanges as a hold of value. Zcash relies on a form of cryptography called a “Zero-Knowledge Succinct Non-Interactive Argument of Knowledge,” or “zk-SNARK,” whereby all of the information in a trade may be shielded or unshielded. This allows one party to reveal to another that information is true without divulging that information.
A shielded transaction is one which encrypts of the information about a transaction (such as its destination and value) but can be verified by the blockchain nonetheless. An unshielded transaction is one which lacks the heightened privacy measures of shielded transactions but is an option within the Zcash blockchain.