The poor and/or the more well-off among the ruled have had it with the rulers and they explode:
In the 2nd century BC in Rome, Tiberius Gracchus, a Senator, thought that the state favored the wealthy too much to stay stable, and attempted to shift things in favor of a broader power base. His popularity with small farmers was tremendous, and appalling to the upper class. His fellow Senators had him killed in 133 BC. The following century is called the Roman Revolution, but it's not the kind of revolution I have in mind here. It was the opposite of a struggle on behalf of the ruled: it was the attempt by one tyrant after another to consolidate all authority in the state into his own hands. Eventually, the man we know as Augustus succeeded in this. The ill-fated attempt at reform by Tiberius Gracchus -- that I would call a revolution.
Spartacus led a revolution of slaves which occupied a large chunk of Italy from 73 to 71 BC. This revolution was crushed, but it was impressive. Later revolutionary movements have named themselves after Spartacus.
To what extent were the Cathar movement and the Hussites and other Medieval uprisings political revolutions, and to what extent were they religious movements? Good question. I don't know.
Martin Luther's rebellion against the Roman Catholic Church was mistaken by many peasants in Germany as a rebellion against all political authority, and they rose up in large numbers, thinking they had Luther's support. On the contrary, Luther was appalled by this disorder and urged the aristocrats to crush the uprising and punish the leaders severely, which they did.
Nevertheless, many people continued to associate Protestantism with revolution. Luther was dead in the 1560's when the mostly-Protestant United Netherlands began to revolt against the very, very Catholic Spain, so we don't know what he would have made of the Dutch Revolution. By 1609 at the latest, the Netherlands were for a practical purposes an independent country. A revolution which succeeded for once! In 1648 Spain finally recognized the Netherlands' independence. After many other nations had.
Between 1640 and 1688, the English deposed and executed King Charles I and secured some civil rights from King William III and his co-ruler Queen Mary II. The term "English Revolution" means different things during this era to different people.
The American Revolution, 1775 to 1783, or 1764 to 1787, or pick pout your own dates, was led by people who thought of themselves as Englishmen who merely insisted upon the constitutional rights which had been granted to Englishmen by William and Mary in 1688.
The French Revolution, 1789 to -- when? -- was led by people who thought they were emulating the American Revolution, but who went much further in overturning the old order, far enough to horrify many American leaders. When did the Revolution end? Was Napolean revolutionary, or did his rule represent reaction and the end of the true revolution? People argue about such things.
The leaders of the Russian revolution (1917-1991) generally admired the French revolution, had various views of the American revolution, and greatly admired Abraham Lincoln.
In 1918, just months after the Russian Revolution began, the German government collapsed, the Kaiser fled, and various local governments were in charge of different parts of Germany. The most powerful single political party was the SPD, the German Social Democratic Party, which had been Karl Marx's party. The SPD did not remain a single party for very long. Some of its members wanted to join the international Revolution -- usually known as the "Russian Revolution." They ended up breaking away from the SPD and renaming themselves, first the Sparticists, and then the KPD, the German Communist Party. The remaining SPD produced many of the leaders of the Weimar republic and the Bundesrepublik, is still around, and has been at times much less Leftist than what Marx had in mind.
I don't know much about the Chinese Revolution. Sorry.
The Vietnamese revolutionaries (mid-19th century to the present) fought occupation first by the French, then by the Japanese, then the French again, then the US until the occupation ended in the 1970's. I'm sure this experience has colored Vietnamese attitudes toward the American and French revolutions.
Fidel Castro, revolutionary leader of Cuba since 1959, greatly admired the US. After overthrowing Batista in 1959 with overwhelming popular support, he tried to ally his government with the US. The US rebuffed Castro, not the other way around, and then and only then did Cuba became the ally of the Soviet Union, and anybody who tries to tell you any different is -- wrong. (I felt like writing something much ruder than "wrong" there.)
Ditto for the Sandanistas, who ruled Nicaragua from 1978 to 1990 and have remained one of several parties in the country since then. Almost ditto: Nicaragua and the US actually were cool from 1979 until Reagan took over from Carter in January 1981 and started the whole Iran-Contra-Cocaine boondoggle.
And there have been many more revolutions! Who's next?