Core: You do radios, this is the 'stereotypical' radio operator. Nowadays they also do tons of stuff like run CP's, signallers for different units, strategic comms, tactical comms, all kinds of stuff. They are the versatile jack of all trades. These are the people who usually get upset if you swear, or say "ack" or "repeat" on the net.
IST: Uber-Nerd, servers and networks. They get spec pay. Sorta. Kinda. Long story. They usually do red (Unencrypted) networks and manage servers, services, and other weird nerd stuff. Some get into programming and development/project jobs, but its rare.
CST: Uber-Nerd, they fix stuff, do vehicle installs, wiring, and that kind of thing. In some places like JSR you'll do lots of black networks, satellites and stuff, in other places you're an electronics fixer-upper. They are a wider net of stuff, but still very technical. They also get spec pay. Sorta. Kinda. Long Story.
LST: These are lineman, they drink from boots. They have secondary duties such as line work, towers, climbing, wiring. They are a very physical trade and when not drinking are some of the hardest working people I know. They are awesome.