also: なんで
Used after nouns and na-adjectives to give a reason or cause. Equivalent to ので, but なので is the form after a noun or na-adjective stem.
See also: ので
Since today is Sunday, the bank is closed.
because of that; that's why; therefore
Colloquial use at the beginning of a sentence, referring back to a previously stated reason. Common in casual conversation.
Because this room is quiet, I can concentrate on studying.
I had a fever yesterday. That's why I skipped school.
だから is a blunter, more casual way to say 'so/therefore' at the start of a sentence, while なので (in its sentence-initial use) sounds a bit softer and more polite.
Combination of the copula だ (in its attributive form な) and the conjunction ので.