We so often hear people[Tamils] speak of "Chanda-t-Tamizh". Men of devotion say that the praises of the lord must be sung in "Chanda-t-Thamizh". "Chanda(m)" is derived from "Chandas".
"Chandas", as I have already said, means the Vedas. Bhagavan says in the Gita that the Vedas are leaves of the pipal tree called Creation-- Chandamsi yasya parnani. Instead of "Veda", the Lord uses the word "Chandas". However, the "Chandas" I am going to speak about does not mean the Vedas but prosody and represents the foot of the Vedapurusa.
The Rgveda and the Samaveda are entirely poetical in form. The Yajurveda consists of both prose and poetry. It is because poetry forms their major part that the Vedas are called Chandas.
The tailor takes your measurement to make your suit. He will not otherwise be able to cut the cloth properly. Similarly, poetry gives form to our thoughts and feelings. Your shirt has to be so many inches wide, so many inches long, isn't so? Similarly, poetry also has its measurement expressed in "feet" and number of syllables. The Sastra that deals with such measurement is "Chandas" and the text on which it is chiefly based is Chanda sutra by Pingala. People who have received initiation into a mantra touch their head with their hand, mentioning the name of the sage associated with the mantra, touch their nose mentioning the chandas and touch their heart mentioning the deity invoked.
All Vedic mantras in verse are Chandas. Non-Vedic poetry is in the form of "slokas". Prose is called "gadya" and poetry "padya". In Tamil, poetry is called "seyyul", in Telugu "padyam". The term chandas also refer to poetic metre(prosody). There is a metre called "Anustubh" in which are composed the Ramayana and the Puranas.
There are rules governing the number of feet in each stanza, and the number of syllables in each foot. The metre "Arya" is based on matras, syllables short and long. Take the word "Rama": the long syllable "Ra" is two matras while the short one "ma" is one matra. There are stanzas in which each foot is determined by the number of syllables, no matter whether they are short or long. Other metres are based on matras.
For a general background, please see here