The PIC18 represents the broad family of devices. Alternatives are PIC12, PIC16, PIC24, dsPIC33, PIC32 and so on.
The F refines the family. Not so much for the PIC18 range, but the letters can represent technology used to implement the chips, voltage and power levels etc.
In general the other letters represent the specific device within the family. To be honest I don't know the specific interpretative of the K. However I would NOT try to use the name to select a part.
Also for other families, in particular the dsPIC33 and PIC24 devices, the first set of digits often represents the memory available and the 2nd set of digits the peripherals that are included but, again, this is often not "codified". For example, the dsPIC33EP5212MU814 (a device I've been using recently), the 512 represents the amount of the main FLASH memory.
Susan