Transactions

Transactions are the most fundamental building blocks of the blockchain system. They represent the transfer of value (cryptocurrency) within the blockchain network between two addresses.

More tangibly, they are represented by small data structures, defined by the blockchain protocol (such as bitcoin or Ethereum), which specifies their attributes (metadata, inputs, outputs, and so on), and model. 

Before broadcasting the transaction, the user sending the funds signs it using their private key (managed by their wallet), and specifies the destination address. Digital signatures and public keys are used to enable network users to validate the transaction, and to check whether the sender has the right to spend the bitcoins held by a specific address.