Pic 12 uses a 12-Bit core.. It can adress less memory, has less instructions and the core is mostly used for smaller (8 pin) pic's
Pic 14 is a 'family' just containing a single pic so i don't really know about it
Pic 16 uses a 14-Bit core. They're the workhorse family of pic's... Capable of adressing quite a bit of memory, some quite powerfull RISC instructions and available from absolute basic pic like 16F84 up to fully packed 16F877...
Never used a Pic17 so ... dunno
Pic 18 uses a 16 bit core. (data still remains 8bit tough) They can get up to 40Mhz, some have up to 65K of program memory. And their instruction set is upgraded. They now have instructions much more suitable for high level compilers (for example, a C compiler can create much more effective code due to the improved assembly set). 18F devices can multiply, divide, and so on...
I still like the 16F familiy the most. If you can get 'em free the 18F rulez, but i buy quite a lot of pic's for projects i make for other ppl. So they have to be cheap AND powerfull and that's just what the 16F series is.