Ohhhh it's danger cannot be over-estimated... or is that under-estimated? I don't know, you can't trust Beverly Crusher when she says these things...
Anyway, the thalaron weapon obviously uses the evil thalaron particle to kill people. It puts on rave show, the radius of which can be limited to a small room, or encompass an entire planet. The effects of this rave caused all exposed to Thalaron particles to start raving, whereupon they eventually trip out so much they die of seizures.
The thalaron weapon was again used on the Romulan Senate. Somehow wiping out the highest tier of Romulan Government paved the way for Shinzon's instatement as Praetor of the Romulan Senate.
Disgruntled at being rejected by his idol Jean-Luc Picard, Shinzon decided to take out his anger on humanity, and more specifically, Earth. His plan was thwarted however; the thalaron generator aboard Shinzon's, ship the Scitimar, was destroyed in an act of self-sacrifice by the notorious he-bot, Commander Data.
Any further research into thalaron weaponry has yet to be discussed or pursued.