Husserl is best known for his extensive use of the notion that the main characteristic of consciousness is that it is always intentional, i.e. directed at some kind of content ("Inhalt"): consciousness is always "consciousness of something." He borrowed the concept of the intentional from Brentano, as can be seen from the latter's Psychologie vom empirischen Standpunkt (Psychology from an Empirical Standpoint). Further, he asserted that studying the flow of consciousness as directed (the act of noesis) at the perceived phenomena (the noemata) yields knowledge of essential structures in reality.
In the last period of his life, Husserl shifted to a more explicitly idealist position, which is best expressed in his Cartesian Meditations (1931). His main work, however, remains Logische Untersuchungen (Logical Investigations; first edition, 1900-1901).
See also: Martin Heidegger