all right, first off, standard deviation is hard to get, true, but once you get it it's a piece of cake. It goes something like this: standard deviation is an approximation used to set a percentage of something contained between a certain distance. For example, if you have a graph as a bell curve, i hope you know what that is, then one standard deviation from the center to both sides would contain 67% of the data. let me explain more: imagine 100 people take a test, you get a graph and put, on the x axis, the score, and the y axis the number of peple with the score. what generally happens is that you have a lot of Cs so if you plot o the X axis A B C D F then you would see that at both ends its really small and then at the middle its really big. All standard deviation is, is the "factor of proportionality", kind of like saying "how spread out were the results". See if ONE standard deviation is really far off from the middle, then that means that the test was pretty even, but if its really close to the middle, then it means most people got Cs and the rest on the sides.
As far as probability goes...well that subject is extremely hard in all levels.
If you really want to know more about standard deviation just send me a message and ill post up some sketches of what i'm talking about.