Can't understand some probability symbols.

#1
For two events, A and B, P(A) =1/2, P(B) =1/3, and P(A B) =1/4. Find P(B | A).

The symbol where is shows a box here is an upsidedown U. Does anyone know what they're asking for here. I P(A upsidedown U B) the same as P(A and B). I'm not sure how to answer the question because I can't interpret the sybmols.

Also, what does P(B|A) stand for? I is this P(B or A). Any help would be appreciated.
 
#3
P(A upsidedown U B) is same as P(A and B), probability of A intersection B, the probability of happening of A and B together.

P(A/B) is the probability of happening of A, given that B has already happend.
P(A/B)=P(A and B)/P(B).