Talk about two ongoing things happening at the same time with "as":
The sky started to get darker as we were walking.
I like to listen to audio-books as I'm driving.
"As" can also come at the beginning of a sentence without changing the meaning:
As we were walking, the sky started to get darker.
"As" is similar to "while". But you can also use "while" when one event was ongoing but the other event wasn't:
Someone called while I was taking a shower.
"As" would not fit perfectly in that sentence, but it wouldn't be completely wrong. So it's safe to think of "while" and "as" as being the same.