Probability is a precise way of describing what may happen. As a branch of mathematics, it investigates the likelihood of complex events, and the way complicated systems work. Looked at another way, probability is also used to describe our degree of belief that something will happen.
Actually, probability theory underlies much of modern science. The laws of genetics are in essence statements about probability. What are the chances that two brown-eyed parents will have a blue-eyed child? How frequent is Tay-Sachs disease in a given population? These genetic questions can only be answered with probabilities.
In physics, probability underlies every modern approach. Theoretical physicists develop complex mathematical models that describe nature, and experimental physicists measure and describe outcomes, often using the tools of probability.
In sciences that rely on sampling and surveys, like some branches of psychology and anthropology, applied probability theory determines methods and guides evaluation. Government uses the tools of probability to attempt to predict the consequences of new laws and regulations.
Gamblers, if they are ever wise, are wise to study probability. In fact, probability as a discipline had its beginnings in the study of games of chance. Probability theory knocks down many beloved superstitions of gamblers, but it also supports some practices that gamblers seem to have always relied upon by instinct.
There are two main approaches to interpreting probability. Some consider probability a way to calculate the tendency of something to occur. For example, the techniques of probability might produce a number that is the likelihood of two people in a classroom having the same birthday. Another way to conceive of probability is as the credence we lend to some unknown outcome. For example, a branch of probability may actually try to investigate which horse is likely to be the favorite at the Kentucky Derby.
The discipline of probability also applies pure concepts to practical matters like the price of a life insurance policy or the weight of a car bumper. Actuaries who match the prices that insurance companies charge to the risks that they are taking must be well versed in the operations of probability. Engineers working on a car, an airplane, or a train must attempt to predict the ways various combinations of stresses will impact their product through various more or less predictable time frames.
The discipline of probability teaches laws and procedures that are useful, if seldom used, in everyday life. At the very least, probability explains why, although someone will certainly win the lottery, you are probably throwing your money away if you buy a ticket.