Flush doors is a name given to doors that are made by using a timber frame which is covered by plywood or MDF or similar board on both the sides of the door. The hollow portion inside the door is usually filled with cardboard or board. These doors are then finished by putting veneer on top or teak ply on top to give it a finishing. These doors have a perfectly flush and joint less surface.
Flush doors have a shutter which is plain in appearance so these doors are easy to maintain as the plain surface does not gather dust. The doors either have veneer skin which is finished using polish or the doors are painted with paint which makes the maintenance easier and all you need to do is wipe clean the doors with dry cloth. In case there is a termite problem in your area, it is recommended to buy termite resistance wood and board for these doors.