Antahkarana is a broad term used in Vedanta and includes manas (mind), buddhi (intellect), chitta (subconscious mind), and ahamkara (egoism). Manas is that aspect of the mind which is constantly in doubt. For example, the thought whether one should buy a car or not etc.
Intellect is the faculty that sheds doubt and takes a firm decision. Chitta is the aspect of the mind which concentrates and enquires into the nature of things. The function of memory belongs to chitta.
Ahamkara is the self-arrogating principle that performs the function of abhimana or egoism. It creates mamata or mine-ness and is the root cause of all suffering.
Mind is under constant sway of three gunas or qualities of prakriti, viz. sattva, rajas and tamas. Sattva represents purity or light; rajas is passion or activity; while tamas is inertia or darkness.
Preponderance of rajas results in constant kshipta or wandering of the mind that becomes a slave of various sensual objects. When the mind is filled with tamas, it abides in ignorance and becomes forgetful. Deep sleep supervenes on account of excess of tamas.
If sattva dominates the mind, thoughts of God, brahm-vichara (enquiry into truth), etc. will manifest. The seeker enters into a meditative mood spontaneously.
Significantly, sattva is always mixed with rajas and tamas and does not exist in isolation. It is important for the seeker to increase the sattvic modifications (like forgiveness, love, mercy etc.) so that he could experience joy, purity, and, ultimately, enlightenment. This can be accomplished by heating the mind in the fire of vairagya (dispassion) and atma-vichara (Self-enquiry).
There is a constant internal fight among the three gunas; between good vrittis (mental modifications) and evil vrittis. This is the warfare between suras (gods) and asuras (demons).