What makes a chant a chant?
What we commonly call "chants" are generically referred to as "plainchants"; Gregorian chants are a specific type of plainchant, those written strictly according to the rules laid down by Pope Gregory the Great, as subsequently modified at various times. Plainchants differ from other music in that a) they are written to be sung unaccompanied, or a capella -- there are no instrumental chants; b) their text is always a religious text and in Latin (always, up to the Reformation, still sometimes in Latin thereafter); c) they do not have a set time signature, like 3/4, 4/4 -- there are rules for part of a verse, but a large part may be sung on one note as long as necessary to fit the text into the musical tune; d) they are not written in the keys which we are familiar with, but they may be in one of the Grecian modes. There are various other rules, such as that the range of the tune never extends as far as an octave, and the technical rules for the construction of a chant tune.

In addition to plainchants, there are also Anglican chants, peculiar to Anglican churches, which are a harmonic form of chant, based on plainchant tunes, which are written in the standard keys and which generally have organ accompaniment.

Some of the great works of Western music have developed out of chants. Many of the chorale preludes of the North German composers, as well as J.S. Bach and his contemporaries, are based on hymn tunes which are themselves derived from ancient chants, like Allein Gott in der Höh' sei Ehr and Vater Unser in Himmelreich.