'Ave Maria' is the Latin prayer commonly translated into English as 'Hail Mary'. The text is medieval, although the first portion, down to '...ventris tui, Jesu.' is very early, while the latter portion is later. The evident Mariolatry of the second part is coherent with this later dating, and the spelling 'Jesu' rather than 'Iesu' is late, too.

Ave Maria,
Gratia plena;
Dominus tecum;
Benedicta tu in mulieribus
Et benedictus fructus ventris tui Jesu.

Sancta Maria
Mater Dei
Ora pro nobis peccatoribus
Nunc et in hora mortis nostrae.

That's as I learned it by heart at secondary school. The usual English text is:

Hail Mary,
Full of grace;
The Lord is with thee;
Blessed art thou among women
And blessed is the fruit of thy womb, Jesus.

Holy Mary,
Mother of God
Pray for us sinners
Now, and at the hour of our death.

The prayer forms part of longer meditations such as Angelus Domini and the Rosary, as well as being used as a general-purpose prayer, particularly in the Roman Catholic church. Although the prayer is thought of as quintessentially Roman, it does in fact get some limited use in other traditions. The musical settings of this prayer are too numerous to mention, and to refer specifically to one composer's work as 'the Ave Maria' tends to lend the impression that only they have set it, or even that they (or their librettist) wrote the prayer itself.