The sides of a building are typically identified as in the attached graphic. Generally, but not always, Incident Command will make it clear what side the Alpha side is, if it is unclear. There may be any number of reasons why the IC adjusts what side is referred to as Alpha. The sides move up the alphabet in a clockwise manner.
As for a division, a division is an individual area of a fire scene. A floor may be a division, a side of a building may be a division. Divisions are often numbered by many fire departments and that is a pre-determined identifier. Division 1 may be the Charlie Side exterior and Division 2 might be the second floor.
This is how I explain it when speaking to the media, or the public, as a fire service public information officer.
