"Rain" is an event that takes place over a period of time, however small. Without further expressed context, both your sentences are stating the same thing. See teddy's reply (#3)A. It rained last night.
( one rain only, it happened and it stopped later, it took part in some time in the night)
B. It was raing last night.
( it kept raining last night.)
No. Both mean that it rained for an indefinite period of time.But both of them actually mean that it rained all the time last night?
If someone asked me "Was it cloudy last night?" I might reply "It must have been. It rained." You could also say "It was raining" with no difference in meaning.Then which one should I use in coversation? For example, if someone asks me: was it cloudy last night? Then can I say "It was raining last night?" Because I want to emphasize the fact.