The type and amount of maintenance required depends on the type of door. Let's take a look some common door materials:
Steel: These doors are very low maintenance, and have excellent insulation properties.
Aluminum: Low maintenance, but not a good insulator. These doors are very light, so are most often used for balcony or screen doors.
Wood: Some maintenance required. Solid wood doors are quite rare nowadays, and would likely be found only in heritage type homes. Modern laminate doors are less prone to warping due to weather and temperature changes. These doors have panels made of wood veneer, with insulation material injected in between. They need to be sealed against humidity, and then painted or stained. Good weather stripping will help make the door air tight, and should be replaced as necessary.
Fibreglass: These doors are very low maintenance. Just wash them with soap, and check the weather stripping once in a while. They are resistant to wear and tear, and are humidity proof.
PVC: These doors are also low maintenance. PVC doors are very resistant to weather damage and corrosion, very sturdy, with steel reinforcement inside, and good insulation properties.