z series= double din, with hard drive
d series= double din, the d2 and d1 didnt have a cd slot if i remember correctly
n series= single din, flip out screen, single cd slot.(what i have)
Since I've owned every Avic Letter, here it goes.
Avic N1 = Original Flip out - One CD/DVD Drive
Avic N2 = Upgrade to N1 - Basically a Software upgrade
Avic N3 = You guessed it. Upgrade to N2
Avic D1 = Original Double DIN - One CD Drive and one DVD Drive - No DVD Decoding hardware built in.
Avic D2 = Must I say it? Ok, Ok. Upgrade to D1 - I hear it can do bluetooth with the correct adapter. Never tried it, don't have a D2.
Avic D3 = Upgrade and stepdown from D2. Upgrade = Faster UI, all buttons on the left side. One CD/DVD Drive. I put one in my 98 to replace the D1 I had in there. D3 = Sucky sucky unless you're into watching DVD's while driving. It supports bluetooth through adapter. It can change button backlighting colors though! yawn....
Avic Z1 = Best Pioneer has to offer. Built-in Hard Disk (30 gig, I believe), one CD/DVD drive, bluetooth, color matching???
Avic Z2 = Upgrade to Z1.
All units can do XM and Sirius as well as XM NavTraffic. I hear the Z series have a much more improved Sirius interface over the other Avic Series.