Matrix is a whole ecosystem of matrix-enabled clients, servers, gateways, application services, bots, etc.
The easiest way to get started is to pick a client that appeals and join #matrix:matrix.org:

https://matrix.org
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This page aims to collect all known Matrix projects - if you want to add a new one (or update an existing one), you can submit a PR to the matrix-doc project on github - the existing projects can be found here - or just let us know in the #matrix:matrix.org room.