The situationist movement is said to have its origin in the Situationist International. The Situationist International was a movement, both artistic and revolutionary, that was influenced both by surrealist art and the ideas of the council communists, who were libertarian Marxists.
It's not always easy to penetrate the jargon of the Situationist International, so it might help to begin with an analogy: Imagine that some sentients are captivated by a massively multi-player online role-playing game -- so captivated that they know little about reality apart from the "reality" they watch while playing the game. The only "being" they know is "appearing". However, every once in a while a player stumbles upon a novel juxtaposition of previously existing elements that makes the player snap out of it for a moment and see that reality exists apart from the game.
According to the Situationist International, people are stuck in a collection of appearances that is like the game described above; they called it "the Spectacle" and said it was the product of capitalism. According to the Situationist International, people become aware of their circumstances when other people make a concerted effort to "jolt" them into awareness. Such a concerted effort is called a constructed situation. People who make constructed situations are situationists.
(If you don't find the analogy above to be helpful, you can try reading the description of situationists in [Section A.4.4 of An Anarchist FAQ]. You can also try reading the [definitions] of the Situationist International; unfortunately the jargon within these definitions is often as intimidating as the jargon they're supposed to explain.)
Like surrealists situationists are interested in bizarre juxtapositions. However, rather than creating new art, situationists novel combinations of pre-existing forms that shows how obsolete the art has become with the arrival of the Spectacle. This use of old art is known as détournment. A simple example of détournement that is still popular is taking a comic strip and replacing the words (of the captions, speech balloons, etc.) with words from something else -- a Marxist critique of capitalist economics, for example. Détournement is obviously a form of propaganda.
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