Where stable is stable. beta is mostly stable. developer is updated one or two times a week and is just a summary of good nightly builds. while nightly is nightly.
So, the nightly channel is literally nightly new stuff.
Aurora channel is probably, "Oh hey, yesterday's nightly stuff worked. Let's add it to Aurora."
Looking through mozilla ftps is a weird thing I used to do more frequently than I should (especially for someone who only ever sticks with stable releases back then).
But Dev updates are daily so far.
I mean Nightly source is continuous update, so not really nightly, but it is compiled nightly based on updates made throughout the day, if I recall correctly.