If you are trying to conserve energy in your home, a good place to start is by installing door weatherstripping. The two kinds of weatherstripping that line the bottom of a door are door shoes and door sweeps.
Door sweeps can be found in the form of a nylon blade that runs along the bottom of the door or it may come in the form of a long narrow brush with densely packed nylon bristles in an aluminum retainer. Some products that come in blade form have multiple blades to ensure a better seal between the bottom of the door and the threshold. The advantage of the door sweep is its ease of installation, as it can be screwed into place on the outside of the door without removing the door. Despite the ease of installation, door sweeps are not as effective at creating a tight, weatherproof seal as door shoes are.
Door shoes are U-shaped nylon strips that cup the bottom of a door. Due to the fact that the blades run along the bottom of the door rather than the outside edge of a door, installation will require removing the door. In addition, depending on the size of the door shoe, you might have to cut a small section off the bottom of the door to accommodate the addition of the door shoe. The advantage of the door shoe is that it creates a better seal than the door sweep and that, generally, it can be less visible than the door sweep.