This video explains what a "random variable" is in Stats, as well as the difference between discrete and continuous random variables. In Stats class, you're almost always using discrete variables when doing probability problems.
The 3 Rules of Probability Distributions
This video covers a very common type of problem that's in every Stats textbook I've ever seen, where they give you a table and ask you "is this a probability distribution". You're supposed to figure it out using the three rules.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)