Why are the PIC "F" models popular?
It's because you're looking at a new chip family. In the 8-bit PICs, the F used to mean flash based (aside from, IIRC, pic16c84 which was the first one) and are therefore more popular than the C series, which had to be erased with UV lights. Some of those didn't even have a window for that, making them one time programmable. 

The actual processor families themselves are divided into numbers from PIC 10 to PIC 32 and dsPIC 33; of which 18 and lower are the well known 8-bit PIC family. I think these numbers once indicated the instruction width, but that changed somewhere (pic18f2550 has 16 bit instructions). The PIC24 I haven't looked closely at, but it's some sort of DSP cousin, and PIC32 isn't even based on the PIC processor architecture but MIPS. You'll find the PIC18 and lower are grouped under 8 bit microcontrollers, while the PIC24 is in the 16 bit category along with dsPIC 30 and 33 (yep, that's a higher number than the PIC32 which is 32-bit). 

In short, the reason you're finding sample code that doesn't quite apply is because Microchip decided to use its well known PIC brand to market nearly all their microcontrollers. The effect you suffer from is known as brand dilution. 

For a comparison, it's like the 10..18 families are like x86 processors from the 386 to the Pentium II, each time adding some instructions but largely similar to program, but the PIC24 is the sudden shift to a dissimilar IA-64 instruction set (Itanium) and the PIC32 is a third party product (like StrongARM). All those were made by Intel, but they avoided this level of confusion by not (at the time) branding the Itanium as a Pentium or Xeon device. AMD, on the other hand, is about to add some confusion with ARM architecture Opteron brand chips.