Probability theories in one form or another have been with humans since the dawn of mankind. Basically, a probability is simply an assumption based on past observable knowledge. For example, if ancient man lived in an area where it was sunny every day for years, then, based on a simple theory of probability, he could reasonably deduce that there was a very high likelihood the next day would be sunny. However, once mathematics equations and models began to be applied in a scientific and accurately measurable way, the modern theory of probability was born. Its father is considered to be the French mathematician Blaise Pascal, who used mathematical formulas and equations to figure out probabilities related to games of chance.
One of the meanings of probability theory is the concept of equally likely outcomes. In this theory, it is assumed that, given fair uniformity of an action, then probability of an outcome is assumed to be given no preference by nature. That is, there is an exactly equal chance of each outcome occurring. For example, a fairly balanced coin will have an exactly equal chance of coming up heads as it will of coming up tails, as nature has no preference, and there are only two possible results.
Another meaning of the theory of probability is the concept known as frequency theory. This states that over a certain number of exact actions, a probability of a certain result occurring can be deduced and expressed as a ratio. This also depends on everything being identical each time the action is performed. For example, throwing a die for an endless number of times will eventually yield a very precise probability that a six will be thrown each time. Over a small number of throws, this probability will be inaccurate, but with every throw, the probability will converge towards the accurate probability figure.
Subjective theory takes into account a person's subjective belief that an event will occur based on their opinions, assumptions and research. This can be used when models such as frequency theory can't be applied. For example, predicting the probability of a large asteroid hitting earth tomorrow would have to be the result of subjective theory, as it is not possible to re-create and test this outcome using frequency theory.