Generally speaking there are two major categories of Cabinet Doors. These are Solid Slab (or plank doors) and 5-piece doors.
The Slab door is simply several boards edge-glued and trimmed to the desired size.
The 5-piece door is more complex but more reliable and more pleasing in appearance.
5-piece cabinet doors are divided into two design categories; Cope and Stick and Mitered.
This post covers the Cope and Stick, 5-piece cabinet door.
Cope & Stick 5 Piece Cabinet Doors
The five pieces in a Cope and Stick door are the four parts of the frame, called Stiles and Rails. The Stiles are the vertical pieces, or sides of the frame and the Rails are the top and bottom pieces. The fifth piece of the 5-piece door is the panel, which the frame surrounds.
The reason the 5-piece door is more reliable than the Slab door is in the way each design responds to changes in relative humidity.
All woods will expand as humidity increases, and as humidity decreases they will contract. An average width Slab Cabinet Door, being between 14 and 20 inches wide, will expand and contract as humidity cycles over the year by as much as three-sixteenths-inches. Some wood types react more and some less, but three-sixteenths-inches is typical for a Slab door.
While this may not sound like much, it is enough to prevent butt-door pairs from closing during high humidity times.
Here is where the 5-piece design comes to the rescue.
In the 5-piece door the panel is allowed to float within the frame, so during panel expansion the panel edges simply move deeper into the Stiles and during contraction the panel slightly pulls back. The Cope and Stick door is designed with a groove for in the stiles so panel movement is accepted without being noticed.
The top drawing shows an end-view of the stile with a contracted wood (low humidity) panel, while the drawing below shows a humidity-expanded panel. Notice that the panel still reacts dimensionally with changes in humidity but in the 5-piece door this panel movement is absorbed by the groove in the stiles. This design keeps the overall width of the door constant as the relative humidity fluctuates.
Cope and Stick doors differ from the other 5-piece design by the method of joining the frame. This drawing shows the difference between the Mitered joint and the Cope and Stick joint. See the post on What is a Mitered Cabinet Door for more info.