Drizzle is a type of liquid precipitation consisting of very small droplets of water falling from low level stratus clouds.
In the UK we're very familiar with drizzle. Our prevailing winds from the Atlantic pick up lots of moisture, and therefore clouds, as they travel over the sea.
To be classed as drizzle, droplets must be less than 0.5mm in diameter. These are larger than the droplets in the clouds, but smaller than raindrops.
For the drizzle to form, the cloud must be fairly low to the ground (usually below 1500ft from the surface), and so it is more likely to occur in hilly areas.