A parachain is a special kind of data structure that is designed for specific applications. It works together with the Relay Chain and can be checked by validators. Think of parachains as parallel chains that run alongside the main Relay Chain. Although they are often built as blockchains, they can take different forms as well.
Parachain
A simpler form of blockchain, which attaches to the security provided by a Relay Chain rather than providing its own. The Relay Chain provides security to attached parachains, but also provides a guarantee of secure message-passing between them