Check out Jerome, Letter 146:
http://www.newadvent.org/fathers/3001146.htm

This was a matter of ongoing discussion all through the middle ages. The dominant view was that the distinction between a presbyter and deacon is one of order, divinely established, while the distinction between a presbyter and a bishop is one of jurisdiction, established by the church, though perhaps with apostolic warrant.

According to the Council of Trent, there are 7 Orders: priest, deacon, subdeacon, acolyte, exorcist, lector, and door-keeper (Session 23). Bishops are a matter of "hierarchy" rather than "order" per se, according to Trent, though Trent does maintain that this hierarchy is divinely ordained.