Understanding Revolution?

Wikipdia's brief definition is a good place to begin:

A revolution is a relatively sudden and absolutely drastic change. This may be a change in the social or political institutions over a relatively short period of time, or a major change in its culture or economy. Some revolutions are led by the majority of the populace of a nation, others by a small band of revolutionaries.

Political revolutions are often characterised by violence, and vast changes in power structures that can often result in further, institutionalised, violence, as in the Russian and French revolutions (with the "Purges" and "the Terror", respectively). A political revolution is the forcible replacement of one set of rulers with another (as happened in France and Russia), while a social revolution is the fundamental change in the social structure of a society, such as the Protestant Reformation or the Renaissance. However, blurring the line between these two categories, most political revolutions wish to carry out social revolutions, and they have basic philosophical or social underpinnings which drive them. The most common revolutions with such underpinnings in the modern world have been liberal revolutions and communist revolutions. In contrast, a coup d'état often seeks to change nothing more than the current ruler.

Some political philosophers regard revolutions as the means of achieving their goals. Most anarchists advocate social revolution as the means of breaking down the structures of government and replacing them with non-hierarchal institutions.

With Marxist communists, there is a split between those who supported the Soviet Union and other so-called 'communist states' and those who were/are critical of those states (some even rejecting them as non-communist, see state capitalism), for example trotskyists.

Social and political revolutions are often "institutionalized" when the ideas, slogans, and personalities of the revolution continue to play a prominent role in a country's political culture, long after the revolution's end. As mentioned, communist nations regularly institutionalize their revolutions to legitimize the actions of their governments. Some non-communist nations, like the United States, France, or Mexico also have institutionalized revolutions, and continue to celebrate the memory of their revolutionary past through holidays, artwork, songs, and other venues.