Moksha (Sanskrit: liberation) or mukti (Sanskrit: release) refers, in general, to liberation from the cycle of death and rebirth. In higher Hindu philosophy, it is seen as a transcendence of phenomenal being, of any sense of consciousness of time, space and causation (Karma). It is not seen as a soteriological goal in the same salvific sense as, say, in a Judeo-Christian context, but signifies dissolution of the sense of "I", or ego, and the overall breakdown of nama-roopa (name-form). It is, in Hinduism, viewed as analogous to nirvana, though Buddhist thought tends to differ with even the Advaita Vedantist reading of liberation. Jainism also believes in moksha.
Hinduism, in support of the idea of moksha, posits the idea of atman and Brahman. A common mistake is to view them, both spoken of as Self, as a monist being of sorts, something possessing substances. In actuality, Hindu scripture like the Upanishads and Bhagvad Gita, and especially the non-dual Hindu school of Advaita Vedanta, say that the Self or Super-Soul is beyond being and non-being, beyond any sense of tangibility and comprehension. Moksha is seen as a final release from one's worldly conception of self, the loosening of the shackle of experiential duality and a re-establishment in one's own fundamental nature, though the nature is seen as ineffable and beyond sensation.
Means to achieve Moksha
There a four yogas (unions) or margs (paths) for the attainment of moksha. They are the ways of selfless work, of self-dissolving love, of absolute discernment, and of 'royal' meditative immersion. Different schools of Hinduism place varying emphasis on one path or other, some of the most famous being the tantric and Yogic practices developed in Hinduism. Today, the two major schools of thought are Advaita Vedanta and Bhakti branches.
Bhakti sees the Self as God, most often a personified monotheistic conception of Vishnu, Shiva or Devi (Mother Goddess). Unlike in Abrahamic traditions, this monotheism does not prevent a Hindu from worship of other gods, beings or teachers, as they are all seen as rays from a single source. The concept is essentially of self-dissolution in love, since the ideal nature of being is seen as that of harmony, euphony, its manifest essence being love. By immersing oneself in the love of God, one's Karmas (good or bad, regardless) slough off, one's illusions about beings decay and 'truth' is soon known and lived.
Vedanta finds itself split three-fold, though the dualist and modified non-dualist schools are primarily associated with the foregoing thought of Bhakti. The most famous today is Advaita Vedanta, a non-dual (i.e. no separation between the individual and reality/God/etc.) perspective which often played the role of Hindu foil to contemporary Buddhist philosophy. In general, it focused on intense meditation and moral realignment, its bedrock being the Upanishads, Brahma Sutras and the teachings of its putative founder, Adi Shankaracharya. Through discernment of the real and the unreal, as a peeling of the layers of an onion, the sadhak (practitioner) would unravel the Maya (illusion) of being and the cosmos to find nothing within, a nothingness which was paradoxically being, and transcendentally beyond both such inadequate descriptions. This was moksha, this was atman and Brahman realized as the substance and void of existential duality.