What is Chanting ?

Recitation of prayers in the form of a mantra with fixed svara(intonation) is called Chanting. It has been passed on to generations via oral tradition. 
 

What is a Mantra ?

Mantra is derived from ‘mananāt trāyate iti mantra’;mananāt = meditating on the meaning, trāyate = it protects, iti = therefore, mantra = it is called mantra. 

Mantra is that which protects if the meaning of the same is meditated upon. 
Example: Gāyatri mantra, saha nā vavatu, asato mā sadgamaya etc.

What is the structure of a Mantra ?

Every mantra has 3 parts. Let us take the Gāyatri mantra for example-
  1. The Devatā : The presiding deity of the mantra.
  2. The Ṛṣi : The vedic sage credited with the mantra.
  3. The Chandas : One of the 6 vedāṅgas; it is the meter in which the mantra is written.   
So, for the Gāyatri mantra the deity is Savitṛ devatā (The śakti[energy] behind the Sun), the ṛṣi is Sage Viśvāmitra and the meter is Gāyatri chandas.
 

What is the purpose of chanting a Mantra ?

There are multiple physical benefits like breath regulation, blood pressure regulation, improvement in concentration, improving awareness/attention and so on. 
The main purpose of chanting is to purify the mind. It helps in replacing old patterns with new and improved patterns.
 

How should one chant ?

One must chant with a rhythmic steady pace like how a horse gallops ! Not too slow, not too fast.
 

How much should one chant ?

At least one Ṛk everyday! One ṛk is only 2 lines long. 
Ṛk is derived from ‘ṛcyate stūyate anayā devatā iti ṛk‘; devatā = the deity, ṛcyate = stūyate = is being praised, anayā = by this, iti = therefore, ṛk = it is called ṛk.
 
Japamāla

What is Japa ? Why is it done ?

Chanting a word or a ṛk multiple times is called japa and it is once again done to purify the mind on a fast track.

Here is how I have been taught…