What is a MUSH?
A MUSH is a type of MU* (Multi User) they are text based environments that look a lot like a command window on a Windows OS. Examples include MUD (Multi User Dungeon) where it plays like a text based version of an MMO. Often a space where you get to travel in a fictional world, fight monsters and level up with multiple stats. A MUSH (Multi User Shared Hallucination) is a type of MU* that focuses more on social interaction and RP in a themed environment. They are still similar in which if you want to do something in a window, you have to type in specific commands in order to do things like chat, move to different rooms.
Compared to your group Skype RPs there is an actual system of interconnected rooms that describe environments for you instead of ad-libbing your own place. By comparison it's a much more established setting with characters that interact in an environment that's managed by an admin.
Here are some simple steps into getting connected, if you have never been to a MUSH before. Here's an explanation and some steps so you can get logged in. Typing in the address in your browser won't work.
There are a few clients, but this may be the best option as it has support for Mac and Linux as well. Try downloading the appropriate version for your computer and starting up the application.
http://www.potatomushclient.com/downloads/
Good luck! +p to chat!