What is the difference between fine threads and corse threads onbolts?

Fine threads are closer together then coarse

Threads per inch. Coarse threads usually have about 2/3 to 3/4 the number of threads per inch than fine threads. A 3/16" bolt or #10 screw will have 32 threads per inch fine and 24 threads per inch coarse.

Fine threads are stronger and more precise, but more expensive to produce and time consuming to fasten.

To add to lowlevel's answer, fine threads can clamp-up (tighten) tighter than coarse threads.

Also, fine threads are less susceptible to vibration loosening, as fine threaded bolts will have more threads yielding while tightening to an adequate pre-load. The resulting plastic deformation on the threads acts as a locking mechanism on the bolted joint.

But, fine threads are easier to strip out (such as a steel bolt screwed into a threaded aluminum hole).

Coarse threads are easier to assemble and need less care; hence they are more commonly used, as they're less likely to be cross-threaded.

Fine threads will, in general, have higher load carrying capacities and higher resistance to vibration loosening. Fine threads are less tolerant to damage and are easily cross-threaded and therefore require more careful assembly.

A lot.

I hate my third grade teacher that told me, "There are no stupid questions."