The Rosary is the traditional devotion in honor of the Mother of God, combining vocal and mental prayer. It is composed of 15 decades which are divided into 3 parts of 5 decades, each of which recall Our Lord's Life, Passion, and Glory.
The Rosary is the prayer which is most pleasing to Our Lady. The word, rosary, means a garland or wreath of roses, thus each prayer said in the Rosary is a spiritual rose offered to our Blessed Mother.
In its present form the Rosary is due to St. Dominic, of the Order of Friars Preachers which he propagated in obedience to a revelation received from the Blessed Virgin in the year 1206. To him we owe also the spread of the devotion which for many centuries has produced the most marvelous results in the Christian world: heresies have been uprooted, morals reformed, misfortunes averted, wars ended.
In keeping with tradition the complete set of Mysteries------15 decades has 150 Hail Marys, the number of Psalms and there are 15 Promises attached to the faithful recitation of the Rosary. The Psalms, found in the Old Testament are hymns of praise to God. One of the best ways to praise God to praise His Holy Mother, Mary, by honoring her in the manner she requested renders homage to the Blessed Trinity Who created her free of Original Sin, the most pure and beautiful of all creatures, above even the Angels. With God there is no such thing as a "coincidence". Thus we know that there is a parallel between 15 Mysteries and 15 Promises and 150 Hail Marys in the decades of the whole Rosary [we do not include the first 3 in the count, which are for an increase in the virtues of faith, hope and love, but the decades themselves, and the number of Psalms: each Hail Mary is a hymn of praise, begun by the Angel Gabriel and added to by Mary's cousin, St. Elizabeth.As for the 15 Promises, they can also be divided into 3 sets of five each. The first five speak of protection and special graces and a desire for heavenly things and not the those of the world. These correspond to the Joyful Mysteries. The next five concern death and our everlasting fate, the promise that is we are faithful to the Rosary we will not have an unprovided for death, in essence. These correspond to the Sorrowful Mysteries. The last five address Heaven, the help of the Saints, becoming the children of God. And these call to mind the Glorious Mysteries.
The Fifteen Promises of Mary to Christians Who Recite the Rosary
1) Whosoever shall faithfully serve Me by the recitation of the Rosary shall receive signal graces.
2) I promise My special protection and the greatest graces to all those who shall recite the Rosary.
3) The Rosary shall be a powerful armor against Hell, it will destroy vice, decrease sin and defeat heresies.
4) It will cause good works to flourish; it will obtain for souls the abundant mercy of God; it will withdraw the hearts of men from the love of the world and its vanities, and will lift them to the desire for Eternal Things. Oh, that souls would sanctify themselves by this means.
5) The soul which recommends itself to Me by the recitation of the Rosary shall not perish.
6) Whosoever shall recite the Rosary devoutly, applying himself to the consideration of its Sacred Mysteries shall never be conquered by misfortune. God will not chastise him in His justice, he shall not perish by an unprovided death; if he be just he shall remain in the grace of God, and become worthy of Eternal Life.
7) Whoever shall have a true devotion for the Rosary shall not die without the Sacraments of the Church.
8) Those who are faithful to recite the Rosary shall have during their life and at their death the Light of God and the plenitude of His Graces; at the moment of death they shall participate in the Merits of the Saints in Paradise.
9) I shall deliver from Purgatory those who have been devoted to the Rosary.
10) The faithful children of the Rosary shall merit a high degree of Glory in Heaven.
11) You shall obtain all you ask of Me by recitation of the Rosary.
12) All those who propagate the Holy Rosary shall be aided by Me in their necessities.
13) I have obtained from My Divine Son that all the advocates of the Rosary shall have for intercessors the entire Celestial Court during their life and at the hour of death.
14) All who recite the Rosary are My Sons, and brothers of My Only Son Jesus Christ.
15) Devotion to My Rosary is a great sign of predestination.
Pray the Rosary
Here is a beautiful webpage I found for praying the Rosary.