In Civilization and Civilization II, the Pyramids are a Wonder of the World. Although they are called by the same name, they have very different effects.
In the first game, the Pyramids convey the power to switch forms of government with no penalty of anarchy, and to switch to forms of government not yet discovered. This could be very handy, as the civlization that built the pyramids could switch to Communism and greatly reduce corruption.
The creators of the game deemed this power to powerful, and in the second game switched the pyramids to working as a granary in every city, which is still very useful.
In both games, the Pyramids require pottery and 200 resource shields to build. In the first game, the pyramids expire with the invention of communism, but in the second, they don't expire at all.
In both games, the pyramids are thought to be a useful thing to build, although obviously for different reasons. In the first, the reduction of communism will allow Civilizations, especially far flung empires, to make more money and get more science. In the second, the pyramids allowed cities to easily grow to much larger populations. However, on higher levels, getting large cities is not always a desirable thing because of Civil Disorder.