The diesel engine was designed to try to simplify the
4-stroke engine. It is basically the same as the
gasoline 4-stroke. Except that on the
intake stroke it only pulls in air. The
compression stroke then packs the air with a ratio of around 24:1 or 250
PSI. Then on the
power stroke fuel(diesel) is injected into the cylinder and the
heat of compression is allowed to ignite the fuel. Eliminating the need of
spark plugs.
Diesel fuel is rated in cetane in stead of octane. Where octane is a measure of how resistant a certain fuel is to burning(a good thing in gasoline engines), cetane is a measure of how "willing" your diesel is to burn(and that's what we want in our diesel engines).
In general diesel engines are more fuel efficient that gasoline engines(I owned a diesel Volkswagen Rabbit that got around 40-50 miles/gallon and it was in pretty sad shape after my parents owned it for 16 yrs). They are less likely to break down completely(Fewer parts. That Rabbit, now back in my parents hands is going on its 20th year and we don't see it dying in the near future). They generally pollute less than gasoline engines, people don't think this is true because of the large amount of soot they produce. And they usually have much higer torque than a gasoline engine of the same size.