Hi all - Newbie here from suburban Philadelphia, Pennsylvania.
I recently purchased a pack of screwdriver bits for my cordless DrillDrivers. Within this set are several different types of bits for driving various screws; Phillips, slotted, torx, and hex. There is also a few bits that look like Phillips but are marked PZ1 and PZ2. I know the PH1 and PH2 = Phillips size 1 and size 2, but what is *Z*??

Pozidriv Insert / Long / Torsion / ACR Bits(PZ0 / PZ1 / PZ2 / PZ3 / PZ4) - Meeng Gang Enterprise Co., Ltd. (http://www.allproducts.com/tool/mgtools/02-screwdriver_bits.html) They may be what's considered a "security set" of bits to make things harder for vandals to take apart.

Ballengerb1 is right, these are security bits.