There are 3 basic types of mounting brackets used for floodlights: 2-inch slipfitters, u-brackets, and 6-inch arms.
1) 2-inch slipfitters: these are the most common type of mounting brackets used for floodlights because of their versatility. Users are able to adjust the floodlights in a horizontal or vertical pattern. The 2-inch slipfitter is fastened onto the light with holes on the light lining up with those on the 2-inch slipfitter. The light with the 2-inch slipfitter attached is then slid onto a round pole of 2 inches to 2 3/8 inches outside diameter. Typically, this is a piece of 2 inch rigid conduit. Or more frequently, a top of a lighting pole has a "tenon" on top of it with these same measurements.
2) u-bracket: this bracket allows for a light to be securely fasted to any solid surface. A u-bracket is a bracket that is shaped like a "U" and is then lag bolted to the solid surface so that your light can be securely fastened. The bracket can then be adjusted up and down to suit your needs. This u-bracket is typically used in lighting applications where the lights are being mounted to a wood pole or to the side of a building.
3) 6 inch arm: this type of bracket is fastened to a light and then fasted to a pole or mounting bracket. The light shines straight down and is not adjustable.