As I sign e-mails, you will find that e-mails have a file attached ending in 7E3D99.asc. This is a signature file, if your client is OpenPGP enabled, then you can import the signature to your client.
Matrix is a communication protocol. You can read a more detailed description here and I have found a video which an introduction below.
The Devon and Cornwall linux user group now have their IRC channel bridged to Matrix, this means that the channel #dcglug can be accessed by anyone who has either an IRC client, Matrix Client or connects to IRC via the web interface for example. This opens up the chat to far more people, which is a good thing.