Extrinsic

An extrinsic (also called transaction) is a call sent to the chain that gets executed.

Some extrinsic is sent by users to be executed on the chain. These are called "signed extrinsics" or "signed transactions", meaning that they are signed with the user's public / private keypair.

Some other extrinsics are called not manually sent by users, but are part of the natural behavior of the chain. These are called "operational extrinsics". Some of these extrinsics are "setting the timestamp" or "validator heartbeats".