In an attempt to make things simple: the order goes (from lowest to highest) priest, bishop, archbishop, cardinal, pope. Local parishes are grouped into regional units called dioceses. A diocese is headed by a bishop, Dioceses are further grouped into ecclesiastical provinces. One diocese in the provence, usually the largest and/or most populous, is call the archdiocese, and it’s bishop is the archbishop for that area. An archbishop runs his own archdiocese, and has some authority over the other bishops in the province.
Cardinal is an honorary title. Cardials are bishops who get to sit in the College of Cardinals. They advise the pope and, when the pope dies, they elect a new one from among their ranks. Most cardinals also run a diocese or archdiocese.
The pope is, of course, the head honcho. He is the bishop of the diocese of Rome (which is called the Holy See) and is the overall uber-boss of everyone else. He is elected for life by the College of Cardinals.
Monsignors are priests who have been given a special honorary title. Usually, they’ve rendered some special service, like founding a parish, or they’ve been a priest for a really long time. Monsignors are nominated by their bishop or archbishop, and are declared such by the pope.
Religous orders, such as the Fransiscans, Benedictines, Jesuits, etc, are a bit more complicated. These priests, nuns, and brothers usually follow the orders of a local bishop, but their real boss the the head of their order. The head of the order reports directly to the pope.