St Dominic (1170 - 1221)
He was born in Castile (part of modern Spain) and became a canon of the cathedral of Osma. He accompanied his bishop (Diego de Azevedo) in a mission of preaching against the Albigensian heresy, which was then strong in southern France. While the official missions lived in formality and splendour, Dominic and Diego lived in extreme poverty, and prepared with great diligence for the debates that they held with their opponents. When the suppression of Albigensianism was undertaken by invasion and war of a particularly savage kind, Dominic continued to try to preach and persuade.
In 1216 he founded the Order of Preachers, dedicated to saving souls by preaching and persuasion. Like the Franciscans, founded a few years before, the Dominicans put great importance on poverty, both of the individual and of the community, and of the need to be involved directly in the world while still living some form of monastic life. At a time when the settled Benedictine monasteries had grown into great and rich institutions, this was a revolutionary and to some a subversive concept. The Friars made a lasting impact on the life of mediaeval Europe, and the Dominicans in particular altered the course of intellectual history by making a well-thought-out and rational response to the new learning that was appearing as long-forgotten thinkers such as Aristotle became known once more in the Christian West. Dominic died at Bologna on 6th August 1221.
http://universalis.comwith reference to the Catholic Encyclopedia as source. Since all the articles are much longer and more detailed in the Catholic Encyclopedia, I'm glad Universalis did a nice summary, but sad to say, omitted the association of St. Dominic and the Rosary.
St. Dominic de Guzman popularized the Marian Psalter in the form we have it today (150 Aves with a Pater after each 10) when Our Lady encouraged him to pray it that way in response to the Albigensian heresy. So associated with the Rosary is St. Dominic that the Rosary is often called the "Dominican Rosary."