cooool forum! 
testing it out and putting this question out there if someone can help me expand my knowledge of this new very exciting world of matrix 
From as far as my understanding goes I think matrix is a client and mastodon the protocol? I see some usernames as xyz@mastodon
No, matrix is a separate protocol, for which there are several clients (we are running Element).
The video here explains it well.
Mastodon is another federated protocol, totally separate from and different to Matrix.
Mastodon is more like a pub sub feed of things happening for a social network. Matrix is the p2p e2e communication layer for the internet 
at least if you believe them.
Relevant sites: matrix and mastodon.
In general, it is interesting to think about the fragmentation of social spaces online over the last decade or so. It only seems to be picking up. There’s always irc for the hardcore people, there’s things like ssb if you’re hardcore + a kiwi, matrix, mastodon (though they serve different purposes), then all the usual centralized stuff… Lots to take in and reflect on, which is one of the reasons that I’ve always liked Matthew’s approach of “communication layer” and the attention they have paid to standards over the years (boring and protracted though that way may seem).