The word "revolution" is used in many different contexts, but it is usually meant to describe an aggressive overthrow of a government structure or social construct or a massive sudden change in societal values. The thing that most sets these events apart is that the change is total. A revolution generally represents a complete turnaround from one way of doing things to another way that is usually diametrically opposite. Most are motivated by the common people deciding to use their advantage to overwhelm their own leadership.
In some societies, oppression can continue for hundreds of years before the people decide to act against it. On a fundamental level, many theorists believe that all governments actually serve at the pleasure of their people, even if it sometimes seems otherwise. When the people finally decide they've had enough, they usually have the power to topple dictators and poor leaders, and sometimes it is remarkably easy to do so. What triggers this isn't always obvious, but when it happens, it can sometimes be sudden and decisive.
Even when revolutionary attempts are unsuccessful, they can potentially lead to significant societal changes. When a group of people decide to make the push for significant change, sometimes the elite will make concessions in order to calm things down. These concessions can represent significant progress, even if they don't represent the total change required by a full revolution. Sometimes, societal change can happen gradually, as failed revolts incrementally push a society into a different direction, until eventually a total overthrow of the former lifestyle is achieved.
The Chinese Revolution and the French Revolution are some very important political examples, but many experts feel the most significant were actually changes in thought and ways of doing things. Examples would be the Italian Renaissance and the Industrial Revolution. These generally represented total massive changes in the way people lived.
People sometimes use the word in relation to technology, and sometimes a new technological device will represent a revolutionary change. A good example might be the invention of the computer or the development of the Internet. Inventions like these can have the power to drastically change a society just as much as any political upheaval. For example, the invention of the automobile changed society on almost every level, affecting many aspects of a person’s life.