Postmodern architecture is a style of architecture that sought to subvert the sterility of orthodox Modern architecture. Heavily based in theory, the style's beginnings can be traced to the 1960s, but it did not become a common language until the 1980s. Postmodernism is highly controversial today, and works in the style are beginning to disappear.
Postmodernism traces its roots to Robert Venturi's highly influential 1966 essay Complexity and Contradiction in Architecture, basically a long-winded argument about why he likes Renaissance and Sullivanesque architecture (and even some of Le Corbusier's work) more than orthodox Modernism and its dogmatic prinicples. Venturi believed Modern architecture recognized complexity and contradiction "insufficiently or inconsistently," idealizing purity over the necessary messiness that architecture was perceived to require. Another important tenet of Postmodern architecture was its embrace of vernacular and kitschy architecture, typified in another Venturi essay (with his partner Denise Scott Brown): Learning from Las Vegas. These writings and others at the same time emphasized that architecture could no longer ignore the "difficult whole," and that decoration was essential to users' enjoyment of a space.
Robert Venturi was one of the earliest practitioners of Postmodern architecture--his 1963 Guild House and 1964 Vanna Venturi House are excellent early examples of the style. Postmodernism gradually took root and became the dominant movement in the 1980s and 1990s. Commissions as large as skyscrapers and as mundane as shopping malls adopted the movement's ideals.
Postmodernism is defined by its return to the forms and ideas of history that Modern architecture abandoned. This varied in its scope--some Postmodern works authentically returned to the language of the past (such as Allan Greenberg's work in the nascent New Classical style); others quoted it through massing, form, or decoration; and a few ironic, campy designs freely played with historical motifs to create absurd compositions. The latter is the oft-derided form that most architects perceive Postmodernism to be today.
Aside from historical quotation, Postmodern architecture can be recognized by a few other characteristics. Polychromy is a significant feature, whether in the bright colors of the Judge Business School or the ever-popular "vaporwave" aesthetics of purples and pinks. Massing tends to be exaggerated and geometric abstraction of common historic forms, and Venturi's idea of the "decorated shed" versus the "duck" has inspired buildings that adhere to both categories. Materiality varies, sometimes brick or stone in more historically informed compositions, but also glass and steel in others. Buildings are organized with a more classical focus, such as the use of axes, though this is commonly abstracted in some way. Ornamentation is frequently employed.
Postmodernism became less popular by the 2000s, but some designs may be considered Postmodern works today. (Michael Graves' IFC Building, built in 1996, looks like a generic 2020s 5-over-1.) Its subsequent movements, such as Deconstructivism and New Classicism, remain common today.