Panel or Flush?
In the broadest terms, interior doors can be classified either as panel or flush doors. A panel door consists of stiles (vertical side pieces) and rails (horizontal pieces) made from solid wood. The spaces between stiles and rails are filled with loose-fitting panels made of solid wood or plywood. The panels fit into grooves in the door stiles and rails.
A flush door features front and back surfaces that are largely flat. A further distinction comes with the construction method. Hollow-core construction consists of a honeycomb-like structure surrounded by a band of solid wood; both faces of the door are covered with veneer, hardboard or thin plywood. The doors are light in weight but are not as durable as solid-core flush doors, which are solid panels of laminated wood, MDF, HDF, or particleboard, covered with wood veneer made of birch, oak, or mahogany.
This distinction between flush and panel doors was originally based on a fundamental difference in how doors were constructed as well as their appearance. However, recent innovations in materials and manufacturing techniques have made it more difficult to distinguish between the two types purely on their looks. A number of manufacturers, for example, offer flush door construction with the traditional detailing of panel doors. In terms of cost, there's some overlap between high-end flush doors and the lower end of the panel door range. This is particularly true when it comes to molded high-density fiberboard doors, which feature crisp surface details similar to those of panel doors and a texture that resembles hardwood.
Choosing panel or flush will ultimately depend on your budget and style. Luckily, there are a multitude of interior door manufacturers to choose from.