Japam is the recitation of a Mantra. Mantra is derived from "Man" which means Mind and "tra" which means tool. Mantra therefore means a tool to control the Mind.
There are two categories of mantras delineated in the shastras. One is the vaidika-mantra, or mantras of the Vedas (such as Gayatri, etc.), and the other is the nama-mantra, or mantras composed of the Lord's names. By definition a vaidika-mantra will possess three aspects, namely a seed letter (such as 'om', 'srim', 'hrim', etc.) which invokes the divinity, a deity to whom the mantra is directed, and a function or offering to the deity (such as 'namah', 'svaha',etc.). Such vaidika-mantras must be received directly from the guru through the process of upadesha, or initiation. This category of mantra has many rules and regulations associated with the chanting of the mantra. It is not that one may chant the mantra at any time or at any place. Prior to the chanting of the mantra one must undergo purification both externally and internally.
This involves bathing (and other procedures from the 'yama' and 'niyama'), meditational postures (asana), breath control to make the mind peaceful (pranayama), withdrawal of the senses from the sense objects (pratyahara), and absorption on the deity (dharana). These six steps of yoga help one to purify oneself, first externally, and then internally. Once situated in purity one may then begin one's mantra sadhana, provided one finds a suitably pure and peaceful location.Such mantras require one to be completely dedicated to their sadhana under the guidance of a qualified guru. These mantras should not be chanted whimsically, as there is all chance of committing mistakes in our sadhana, resulting in us incuring negative reactions.Nama-mantras are quite different from the vaidika-mantras, as they are composed only of the Lord's name.
Such mantras carry the exact same potency as the vaidika mantras, but in a manner accessable to all. When we analyse the power source of all mantras, we will come to understand it is the name alone which empowers the mantra. For example, one may take a mantra such as 'om namo bhagavate vasudevaya'. If one were to just chant 'om namo, om namo, om namo', one will not receive the effect of the mantra. On the other hand, if one were to just chant 'vasudeva, vasudeva, vasudeva', one would receive the benefit of the mantra, for it is Vasudeva's name which empowers the mantra. The full potency of the Lord is present within His holy name:There are no restrictions (niyamas) for chanting the Lord's name; neither must one take consideration of the time or circumstances."The nama-mantras are the Lord's special mercy to the fallen souls of the Kali-yuga. Those of us with no proper spiritual qualification can still attain the Lord and associate with Him through His divine name.
There are three ways by which one can do Japam
Ucchahi - loud recitation - less effective
Upamshu - soft recitation - more effective
Manasikam - within the mind - most effective.
Benefits of Japam
Physical - to energize the system (to tone up the system)
Mental - to remove depression/obstacles/mental blocks.
Spiritual - self-development/specific areas of growth (strengthening the inner-personality by projecting positive powers inwards; over-coming/eliminating negative traits)