LA and CA.
LA - Low altitude alert. If the aircraft is on approach, the radar controller is required to call the tower on the landline and let him know of it. Sometimes, this can be inaccurate because the radar measures the LA based on rate of descent. If there is a high rate of descent, the LA warning will go off (and you'll see a red LA next to the aircraft along with that alarm).
CA - collision alert. This will only occur (at least in my experiences) when two returns break the separation minimums. However, a primary target will not trigger anything. The aircraft must have the transponder on (even if it isn't squawking mode C). Of course, the little red "CA" will show up next to the two returns.
These alarms can be silenced, so when the controller advises "Maintain visual separation," and/or uses the "converging rule" and the aircraft get close, they can silence the alarms so they don't become annoying.
Hope this helps. I'm LH can chime in with much more accuracy than I.