The difference between theoretical and experimental probabilities. 
Theoretical - theory - in the head - idea.
Experimental - from the past experiment.

The idea you need to know is that with time experiment will become theoretical.  The probabilities will match.  e.g. flipping a head (should be 50%)...  There are two sides on a coin one is a head therefore in theory it's 1/2 chance. 

The proper notation is to save time.   P(head) = 0.5   this means "the probability of a head is 50%"

Card Games - we think no jokers so 52 cards.  Four suits (hearts/clubs/spades/diamonds).   Picture cards (king/queen/jack).