Drizzle is a term used for a very light, fine rain. The drops tend to be very small and not very dense. Mist is similar to drizzle, only the number of drops in any given area is greater. This is usually the case on heavy fog days.
As for showers, "isolated" and "spot" showers mean just a couple of showers spread over a wide area, and that most of the area is dry. "Scattered" showers mean more shower coverage -- maybe 30-60 percent coverage of the area. "Frequent" or "widespread" showers mean a large area of coverage.