I have some homework that I cannot figure out because of this formula:

$\displaystyle P(B|A) = \frac{P(A and B)}{P(A)}$

I don't exactly understand the above formula for calculating the probability of B given A. (So basically I don't understand what the probability of "B given A" means and when to use it.) If someone could explain this to me, it would be greatly appreciated. (Bigsmile)