German Idealism is a philosophical movement centered in Germany during the Age of Enlightenment of the late 18th and early 19th Century. It developed out of the work of Immanuel Kant and is closely linked with the Romanticism movement. It is sometimes referred to as Kantianism (although that more correctly also involves acceptance of Kant's ethical and epistemological views).
Other than Kant himself, the main contributors (who all had their own versions of Kant's theory, some close in nature and some quite distinct) were Johann Gottlieb Fichte, Friedrich Schelling, Georg Wilhelm Friedrich Hegel and (arguably) Arthur Schopenhauer, and additionally Friedrich Heinrich Jacobi (1743 - 1819), Gottlob Ernst Schulze (1761 - 1833), Karl Leonhard Reinhold (1757 - 1823) and Friedrich Schleiermacher (1768 - 1834). Although essentially a German movement, the Swiss-French writer and philosopher Madame de StaŽl (1766 - 1817) introduced (in her famous book "De l'Allemagne") the works of Kant and the German Idealists to French thinkers, who were still largely under the influence of John Locke at that time.
In general terms, Idealism is the theory that fundamental reality is made up of ideas or thoughts. It holds that the only thing actually knowable is consciousness (or mental entities), and that we can never really be sure that matter or anything in the outside world actually exists. The concept of Idealism arguably dates back to Plato, and reached a peak with the pure Idealism of Bishop George Berkeley in the early 18th Century. See the section on the doctrine of Idealism for more details.
The German Idealists, however, were dissatisfied with Berkeley's rather naive formulation. In the 1780s and 1790s, Immanuel Kant tried to bridge the two dominant philosophical schools of the 18th Century: Rationalism (which held that knowledge could be attained by reason alone, a priori), and Empiricism (which held that knowledge could be arrived at only through the senses, a posteriori). Kant's Transcendental Idealism claims that we know more than Berkeley's ideas in our minds, in that we also directly know of at least the possibility of "noumena" ("things-in-themselves"), which are both empirically and transcendentally real even if they cannot be directly and immediately known. The actual "phenomena" which we perceive and which we think we know are really just the way things appear to us and not necessarily real.
Other German philosophers of the time used Kant's work as a starting point, adding in their own interpretations and biases. As a movement, it was not one of agreement (although there was some common ground), and each successive contributor rejected at least some of the theories of their predecessors. Many of the German Idealists who followed Kant, effectively tried to reverse Kant's refutation of all speculative theology and reinstate notions of faith and belief in their explanations of what exists beyond experience, a trend which was continued later in the 19th Century by the American Transcendentalists.
Jacobi, although in agreement with Kant that the objective thing-in-itself cannot be directly known, tried to legitimized belief and its theological associations by presenting the external world as an object of faith, even if logically unproven. Schulze tried to use Kant's's own reasoning to disprove the existence of the "thing-in-itself", arguing that it cannot be the cause of an idea or image of a thing in the mind. Following from Schulze's criticism of the notion of a "thing-in-itself", Fichte asserted that there is no external thing-in-itself that produces the ideas, but our representations, ideas or mental images are merely the productions of our ego, or "knowing subject". Schelling's view was that the ideas or mental images in the mind are identical to the extended objects which are external to the mind ("absolute identity"), so that there is no difference between the subjective and the objective. Schleiermacher's variation was that the ideal and the real do not have a productive or causal effect on each other, but are united and manifested in the transcendental entity which is God.
Another German Idealist, G. W. F. Hegel, claimed that pure abstract thought (as in Kant's formulations) is limited and leads to unsolvable contradictions. In order to overcome these shortcomings, Hegel introduced the integral importance of history and of the "Other" person in the awakening of self-consciousness. In the process, he established a whole new movement of Hegelianism, which in turn was hugely influential in the later development of Continental Philosophy, Marxism and (by virtue of its opposition to Hegel) Analytic Philosophy.
Schopenhauer claimed that Kant's noumenon is the same as Will, or at least that Will is the most immediate manifestation of the noumenon that we can experience. He saw the "will-to-life" (a fundamental drive intertwined with desire) as the driving force of the world, prior to thought and even prior to being.
Schopenhauer's criticisms of the later German Idealists is seen by some as a sort of "back to Kant" movement, giving impetus to a Neo-Kantianism movement in the mid-19th and into the 20th Century, which yielded the Kantian analyses of such German philosophers as Kuno Fischer (1824 - 1907), Friedrich Lange (1828 - 1875), Hermann Cohen (1842 - 1918), Paul Natorp (1854 - 1924), Nicolai Hartmann (1882 - 1950), Ernst Cassirer (1874 - 1945), Wilhelm Windelband (1848 - 1915), Heinrich Rickert (1863 - 1936) and Ernst Troeltsch (1865 - 1923).
Also in the mid-19th Century to the early 20th Century, a movement which became known as British Idealism revived interest in the works of Kant and Hegel. The leading figures in the movement were T. H. Green (1836 - 1882), F. H. Bradley (1846 - 1924), Bernard Bosanquet (1848 - 1923), J. M. E. McTaggart (1866 - 1925), H. H. Joachim (1868 - 1938) and J. H. Muirhead (1855 - 1940).