Early in the film, the principal character mentions Monet's "vanilla sky" in a painting.
Warning: Wikipedia contains spoilers.
Throughout the movie, the skies are all "vanilla" just like in Monet's painting - in fact, a little too like the painting. That is the only giveaway that we're in a fantasy movie about simulated reality, because it plays it straight, about modern life, a successful womanizer who becomes the victim of a Fatal Attraction.
The simulated reality is the creation of a company called Life Extension (LE) that has sold the protagonist David (played by Cruise) a "lucid dream", which the company calls the "cryonic union of science and entertainment." The subject's body is kept frozen, but the mind is left to roam free with no recollection of the subject's own death. If something goes wrong with the simulated reality, the company can send technical support (played by Noah Taylor) to the subject.
While the issue of whether what one is experiencing is reality or simply being fed to the person is a critical point used to support the ending, the issue and the question (and all that it entails) is only peripherally raised in the film. While it is somewhat more emphasized in the movie EXistenZ, the subject is most clearly raised in The Matrix, and is touched upon quite heavily in the film Total Recall.
Various people have speculated that the picture depicted in the film is either Monet's "Falaises et Voiliers a Pourville", or even one of the series named variously "Wheat Field and Cypress Trees" / "Yellow Wheat and Cypress" / "Paysage avec Cypres" by Vincent Van Gogh.
See also: life extension