Phoenix,
I can't remember too much about TCAs now as the aircraft I fly aren't fitted with it, so all I can recall is from my exam studies several years ago now. I apologise then if some facts aren't entirely correct but you'll get the principle.
TCAS provides traffic information on other
transponding (i.e. they have a radar signature from a gadget in the cockpit) aircraft, and plots their with respect to you. It projects a kind of "protective bubble" around the aircraft based on distance and time. Any targtes likely to enter this zone produce a "traffic traffic" auidble warning in American robotic computer speak and show up as a coloured symbol on the TCAS screen - can't remember what colour or symbol though!). These are called "traffic advisories". Traffic not in the bubble but near by are called "proximates" and also given a symbol and colour.
If the target gets closer then the system will issue a "resolution advisory" when the time to collision has got really quite cheeky. It will command a climb or descent or tell you to go faster if already climbing or descending. RA's are given another symbol and colour, once again I can't remember what!
When all the excitement has passed, "clear conflict" is given.
Hope that helps
MM
