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Matrix: Protocol (like SMTP/IMAP)

Element: The most popular client (like Thunderbird)

Element X: Rewrite of Element based on Rust and native UI components

Synapse: Most popular Matrix server (like Dovecot)

Element Call: Video/Audio conferencing (like Jitsi)

Element Server Suite (ESS): Paid SaaS solution that wraps all of the mentioned components into a simple turnkey solution.

Matrix is the protocol, Element X is one of many clients for the Matrix protocol.

Element is a client, matrix is a protocol, synapse is a mainline backend server. There are other clients (front-ends) and servers available on top of the matrix protocol. Hope that helps.