The above reference is better, but a quick explanation is SP means single poly and SPN means single poly with a nylon overcoat. HPN would be heavy poly with a nylon overcoat. There are also different meanings for heavy. The single build wire is dipped only once, but some wires are dipped twice or three times to build the insulation thickness.
Basically, for single coil strat pickups you would probably like the heavy coated versions better. Most of the original strat pickups used heavy formvar. For humbuckers you would probably like the single coated versions better. But the single coated wire can also be used to wind strat coils, the ones from the mid 60's used PE. If it is plain enamel wire the description will say so and the price will be much higher so it is pretty easy to tell. There is plenty of discussion about the effects of different wire types all over the forum, so do look at some of the threads or search for the term you are curious about like PE or SPN. I would say for starting out, the most versatile overall and cheapest would be 42 SPN.