Opera 101 - what is an "A" cast and a "B" cast?

I haven't done and "edutainment" post in a while...
Opera is different than musical theater when it comes to the amount of performances per week.
Musical theater may have 8-10 perfs a week, with some days having matinee/evening performance on the same day.
Opera often only has 4 or 5 per week, with 2 casts alternating - meaning, if the opera opens on a Thursday evening with the "A" cast, then Friday is the "B", Sat is the "A", usw.

When the opera is reviewed, savvy writers refer to which cast they saw - "A" or "B", and yes, it sometimes does make a difference, although we all have heard of train wreck A casts getting blown away by the B cast.

For your viewing pleasure, a British joke that "gets" it: