Samkhya means to enumerate, analyse or count and it is a school of Hindu philosophy founded by sage Kapila. It deals with the analysis and discernment of the different properties of consciousness and matter([wiki]purusha[/wiki] and [wiki]prakriti[/wiki]) and their interactions with one another.
Samkhya philosophy underpins Yoga philosophy and practice entirely. It is also the foundation of Ayurveda and much of Hinduism and is considered by scholars to be amongst the oldest philosophies in the world(if not the oldest itself). It can be traced in a rudimentary form to the [wiki]Vedas[/wiki]. Later, it is found in a more developed form in the [I]Bhagvad Gita[/I] which contains a chapter on Samkhya. The former are considered to be theistic forms of Samkhya, later a formal and atheistic school of Samkhya develops. The [I]Samkhyakarika [/I]by Isvarkrishna is the oldest extant text of the formal school. The Samkhyakarika is a philosophical/metaphysical treatise which explains using rigorous logic the Samkhya theories of physics, cosmology and psychology.
Most of the well known concepts in Yoga philosophy and much of the Yogic vocabulary comes from Samkhya theories. For example the concept of yugas or cycles of time, karma, reincarnation and spiritual evolution comes from the Samkhya theory of cosmology; the apparatus of the mind-body consisting of 5 senses(jnanaindriyas), 5 motor organs(karmaindriyas), 5 subtle elements(tanmatras), 5 material elements(mahabhuttas), mind(manas), false ego(ahamkara), intellect(buddhi) and subtle body comes from Samkhya theory of psychology. The concepts of prakriti or original matter, and the gunas or fundamental forces within matter, the 5 pranas or 5 vital life forces, comes from Samkhya theories of physics. In short Samkhya is beating at the very heart of Yoga. Indeed, it is the master system behind Yoga.