"It is spitting" This is for rain that is very light and for a short time.
"It is drizzling". This is for continuous rain for a long time. This is most common in fall.
"It is bucketing down". This is for strong rain - like someone is emptying buckets of water on your head.
"It is raining cats and dogs". This for the heaviest rain. It is from very old English. Many years ago cats and dogs used to live in the straw roofs of buildings. When the straw got wet, the cats and dogs would slip and fall down. It looked like it was raining cats and dogs!