The second book of the Khuddaka Nikāya of the Sutta Pitaka.
It is probably a later anthology than the Thera-Therī-Gāthā, and its earliest mention by name is in the Milinda-pañha (p.408).
It includes gāthas collected together from various books in the Canon, but contains hardly any from the Jātaka collection, or directly derived from the Sutta Nipāta.
The present text of the Dhammapada contains four hundred and twenty-three verses divided into twenty-six vaggas.
So far, five recensions of the Dhammapada have been discovered. (For details see Law: Pāli Lit., pp.215f).
A commentary on it exists called the Dhammapadatthakathā.