Type: what the particle is (so you can change things from, say, WOOD to BMTL)
Life: varies, but for a lot of elements it's how long they stay on the screen (eg. BRAY)
CType: again it varies, but it usually determines stuff like what CLNE clones, what ICE melts into, etc.
X: the particle's position on the screen, from left to right
Y: the particle's position up and down
VX: the particle's sideways speed (negative to go left)
VY: the particle's vertical speed (negative to go up). Both of these only work for stuff like PHOT or NEUT
Temp: temperature
Flags: not sure about this one
tmp: again it varies, but generally it stores information when Life is already used. Does stuff like setting the length of beams created by CRAY
tmp2: same as above
Dcolour: basically a harder to use DECO tool, you have to put in the colour you want as a hexadecimal (including opacity)
Not sure about the last two

It seems pavg[1] is used by VIRS to determine when it dies, pavg[0] for when it's cured.

PIPE allegedly uses them, but I'm not sure how.

Good-to-know stuff. Flags might have something to do with updating moving particles. I am but an ignorant user.