Hazardous Substances

No I'm not gonna poison anyone, I read all the warning stickies. Part of this thread is just personal interest, but some will be invariably used in my writing, as much of my research usually is. I could probably just Google most of these questions but I suck at internet.

1: How are toxins and hazardous substances usually categorized? Mode of action, similarities in chemical structure, etc? Do biological effects have a correlation with chemical similarities?

2: Which hazardous substances/classes thereof kill the fastest? Which kill faster than say....severe abdominal trauma? That's a bit vague....how about an arrow to the pancreas?

3: Which have the lowest median lethal dose? Can a poison have a low LD50 but kill slowly, or a high one yet kill quickly?

Any help greatly appreciated!