Ethereum is both a platform and a protocol: it is a place where something (accounts, code, state) is hosted and a set of rules for communicating between those things. Smart contracts are to Ethereum the platform as transactions are to Ethereum the protocol.
Transactions therefore sit at the very core of the conceptual model. They have some important properties:
They compute values, normally based on state. A read transaction encodes a set of instructions to derive values from state. A write transaction encodes a set of instructions to ...
