Tech

Tech are responsible for, wait for it, the "tech" at conventions :-) .. this includes all the lighting, sound and pretty much anything with a plug on it aside from computers, and the art show lighting

Sound
Tech are responsible for setting up PA (Public Address) systems in rooms that need it. And for making sure the microphones work, that the amp is switched on and connected to everything, and that there's someone at the sounddesk to twiddle knobs and slide sliders!

Tech may also be responsible for ensuring there is some sort of playback device, such as a CD player or iPod dock, for panels/items that need it.

In general, a room of up to 50 people probably doesn't need microphones and PA equipment, unless items are being recorded, or there's a particularly quiet speaker, or they need the PA for music or playback of audio (such as in a video room, or for video conferencing, or recorded presentations).

Lighting
Very few rooms (usually just the main hall) get lighting. Again tech are responsible for setting it up, tearing it down, and lighting the various events in the main hall.

Audio/Visual
Tech are also responsible for video projection, DVD players, TV screens, MP3 playing, hooking up laptops to video projectors etc.

Disco/Band
And if there's a disco and it doesn't bring its own lighting/PA, then tech have to assist with that.

Convention Radio System
Tech are responsible for ordering and setting up (and taking down and returning) the hand-held walkie-talkies used by the convention organisers, Ops, Gophers/Stewards and anyone else. These radios are also known as "wallyphones" (short for walkie-talkie-telephone, and also because you look foolish talking "to yourself" when you've got one) In the UK we usually hire them from because they give us a good price, and because they come with a licence to use them in the area of the con, their own battery chargers, headsets etc. and are sufficiently rugged and reliable. We've looked at other options and everytime we come back to hiring them in.