Phenomenology is a philosophical trend that takes the intuitive sense of conscious experience -- the "about-ness" of something -- and attempts to extract or describe its fundamental essence. When I want or hate something, what is the exact relationship between me and it, independent of external factors? The field largely derives from the turn-of-the-century work of German Jewish philosopher Edmund Husserl, and has been discussed throughout most of the 20th century by thinkers including Martin Heidegger, Maurice Merleau-Ponty, Max Scheler, Hannah Arendt, and Emmanuel Levinas.
Characteristic of phenomenology are extended discussions of intentionality -- a quality presumably unique to consciousness that distinguishes it from aconscious materials. Endemic to older discussions of phenomenology is mind/body dualism, the notion that psychical (mind-based) acts are somehow ontologically different than physical acts, an idea that modern cognitive science has shown to be false. Many of the questions addressed by phenomenology have been inherited and improved upon by the contemporary and more scientifically rigorous field of consciousness studies.
The field of phenomenology began in 1901 when Husserl published the Logical Investigations, his first major work, which analyzed the relationships between mental acts and their external referents. For example, one might hate or love an object or ideal. In later works he made distinctions between intentional acts (noesis) and the targeted objects (noemata). In attempting to get to the "core" of intentionality, he took examples and stripped away as much inessential detail as possible, such as assumptions about the external world and the incidental qualities of the noemata.
Today we use experimental research and the biological sciences to determine the details of the relationship between the thinker and the objects he or she is thinking about. Philosophy provided a stopgap way of investigating the issue when it was difficult to structure detailed experiments around these relationships. The facts we have determined about these relationships, such as the details of human symbolic representation, still require a lot of working out, and philosophy helps frame which experiments may be useful. Nevertheless, phenomenology represents a typical area of philosophy that has been eroded by the march of empirical science.