Monsignor is an ecclesiastical honorific for clergy of the Roman Catholic Church. (For the Anglican Church, see honorary Canons.) The word derives from the French Mon Seigneur, meaning “my lord”. In English, it is abbreviated Mgr., Msgr. or Mons.

“Monsignor” is a form of address which is connected with appointments to certain positions (most of which are honorary). The honor is bestowed upon priests directly by the Holy Father, most often in consultation with the local bishop. In general, no more than 10% of a particular diocese’s priests may be named monsignor.[citation needed]

Monsignor (or its equivalent) is the simple style for bishops (including archbishops) who are not cardinals, as opposed to the formal style of Your Excellency or “Your Grace,” or the simple and informal prefix of “Bishop” commonly used in the United States. In English-speaking countries, however, usually only priests who have been included in the Pontifical Family are called “Monsignor”; though Bishops do have the same privilege, it is rarely ever practiced. Appointment as an honorary prelate is simply the award of a title with certain privileges, including the style of Monsignor (as opposed to an ordination as a bishop, which confers a sacrament and enables the person to confer the sacrament of Holy Orders in turn).

http://en.wikipedia.org/wiki/Monsignor