Doors that lead to the exterior of the home get dirty from the outside elements. The amount of dirt may vary depending on if there is an overhanging cover that protects the door from the weather elements. Many of these doors have groove or edge if there is glass embedded in the door that collect a lot of dirt. Be sure to clean the inside and outside of these doors. If you have wooden doors, you should also periodically stain the doors and polish the doors to keep the original color which bleeds due to sunlight and dries out.
Start with a duster to dust off the top and sides of the door time. Do the same for the door itself, especially if there are cutouts or ridges where dust can collect.
Then take an all-purpose cleaner and either a paper towel or towel spray and wipe down the trim pieces and the door, especially near the edges where the door swings open and closed, as that is where most hand prints and other stains may be.
Don’t forget to do the same with the door handles and knobs, as this is where germs can get transferred as people use the handles. Here is a quick video from eHow.