The Angelus Domini, shortened to "the Angelus," is the ringing of the church bell -- in three groups of three chimes with a pause in between each group, followed by 9 consecutive strokes -- at 6AM, Noon, and 6PM roughly, and its associated prayers, which spring from the monastic practice of praying the tres orationes at Matins, Prime and Compline. While the monastics said their prayers at the sound of the Angelus Bell, the faithful would stop what they were doing and say 3 Hail Marys in honor of the Incarnation. Later, since at least A.D. 1612, verses were added to these Hail Marys such that we get the form of the Angelus we have today (see below). During Paschaltide (the Easter Season), the humbling Angelus prayer below is replaced with the more celebratory, joyous Regina Coeli prayer at the direction of Pope Benedict XIV in 1742.

Some of the earliest bells used for this purpose, dating to the 13th and 14th centuries, still survive and are engraved with inscriptions attesting to their purpose. Some of these inscriptions are (from the Catholic Encyclopedia):

Ave Maria, gratia plena, Dominus tecum (Hail Mary, full of grace, the Lord is with thee)

Dulcis instar mellis campana vocor Gabrielis (I am sweet as honey, and am called Gabriel's bell)

Ecce Gabrielis sonat hæc campana fidelis (Behold this bell of faithful Gabriel sounds)

Missi de coelis nomen habeo Gabrielis (I bear the name of Gabriel sent from heaven)

Missus vero pie Gabriel fert læta Mariæ (Gabriel the messenger bears joyous tidings to holy Mary)

O Rex Gloriæ Veni Cum Pace (O King of Glory, Come with Peace)

Sadly, there are few places where the Angelus is still rung consistently. Vatican City, of course, still chimes the Angelus, and so do various institutions in the Republic of Ireland, but in this latter case, the cause of "diversity" is challenging the practice. Christendom is losing once again.
 
  

The Prayers of the Angelus
 

The Angelus
During Paschaltide, this prayer, said kneeling, is replaced by the Regina Coeli (see below).

V		The Angel of the Lord declared unto Mary.
R		And she conceived of the Holy Spirit.
All		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 in the hour of our death.
V		Behold the handmaid of the Lord.
R		Be it done unto me according to thy word.
All		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 in the hour of our death.
V		And the Word was made Flesh.
R		And dwelt among us.
All		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 in the hour of our death.
V		Pray for us, O Holy Mother of God.
R		That we may be made worthy of the promises of Christ.
V		Let us pray. Pour forth, we beseech Thee, O Lord, Thy grace into our hearts; that, we to whom the Incarnation of Christ, Thy Son, was made known by the message of an Angel, may by His Passion and Cross, be brought to the glory of His Resurrection. Through the same Christ our Lord.
All		Amen.

Latin Version:

V		Angelus Domini nuntiavit Mariae;
R		Et concepit de Spiritu Sancto.
All		Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fructus ventris tui, Iesus. Sancta Maria, Mater Dei, ora pro nobis peccatoribus, nunc, et in hora mortis nostrae. Amen.
V		Ecce ancilla Domini.
R		Fiat mihi secundum verbum tuum.
All		Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fructus ventris tui, Iesus. Sancta Maria, Mater Dei, ora pro nobis peccatoribus, nunc, et in hora mortis nostrae. Amen.
V		Et Verbum caro factum est.
R		Et habitavit in nobis.
All		Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fructus ventris tui, Iesus. Sancta Maria, Mater Dei, ora pro nobis peccatoribus, nunc, et in hora mortis nostrae. Amen.
V		Ora pro nobis, sancta Dei Genetrix.
R		Ut digni efficiamur promissionibus Christi.
V		Oremus.Gratiam tuam, quaesumus, Domine, mentibus nostris infunde; ut qui, Angelo nuntiante, Christi Filii tui incarnationem cognovimus, per passionem eius et crucem, ad resurrectionis gloriam perducamur. Per eundem Christum Dominum nostrum.
All		Amen.

 
 
Regina Coeli
This prayer, said standing, is used to replace the Angelus during Paschaltide.

 

All		Queen of Heaven rejoice, alleluia: For He whom you merited to bear, alleluia, Has risen as He said, alleluia. Pray for us to God, alleluia.
V		Rejoice and be glad, O Virgin Mary, alleluia.
R		Because the Lord is truly risen, alleluia.
V		Let us pray : O God, who by the Resurrection of Thy Son, our Lord Jesus Christ, granted joy to the whole world: grant we beg Thee, that through the intercession of the Virgin Mary, His Mother, we may lay hold of the joys of eternal life. Through the same Christ our Lord.
R		Amen.

Latin Version:

All		Regina coeli, laetare, alleluia: Quia quem meruisti portare, alleluia. Resurrexit sicut dixit, alleluia. Ora pro nobis Deum, alleluia.
V		Gaude et laetare, Virgo Maria, Alleluia,
R		Quia surrexit Dominus vere, alleluia.
V		Oremus : Deus qui per resurrectionem Filii tui, Domini nostri Iesu Christi, mundum laetificare dignatus es: praesta, quaesumus, ut per eius Genetricem Virginem Mariam, perpetuae capiamus gaudia vitae. Per eundem Christum Dominum nostrum.
R		Amen.