This specifies how the rim will be mounted to the board. Front mount means the rim will bolt directly into the front of the backboard. Rear mount means the rim will bolt to the backboard, via a mounting bracket on the back of the backboard and hang out from underneath. Rims are either front mount or rear mount. One should purchase a rim to fit the type of backboard they have.
Rims are either stationary or breakaway.
Having a break-away rim is highly recommended in facilities where hard play is expected, such as high schools and recreation areas. Breakaway rims take the stress from the backboard and rim and transfer it to the springs of the breakaway system. This helps prevent shattered glass, bent rims and broken backboards. Some rims are offered as double rims. This type has two steel hoops one on top of another to provide added strength and help prevent bent rims.