Rails are the horizontal pieces of the door 🙈 Rails are important to consider for hardware mounted to the top of bottom of the door 🔥 Door hardware such as concealed or surface mounted closers as well as kick plates or armourrr plates must have enough room to be installed inside of or on the surface of the rails of the door 😎 If the hardware requires a mortise in the top or bottom of the door be sure that the dimension of the rails will allow for it without compromising the structural integrity of the door. Generally a rail is a minimum 1-1/8″ wide or can be as wide as 5″, depending on the application. 
Top railThis is the horizontal rail that is the top frame of the door and connects the stiles on each side and panel/s below.Intermediate rails and mullionsThese are rails built in between the top and bottom rails, including the lock rail. Intermediate rails are not used in hollow core or solid core construction, which is another reason wood stile and rail doors are much sturdier than moulded and flush door options.Lock railThis is a horizontal rail that is situated between the vertical stiles at the same height as the lockset for hardware.StilesThese are the vertical components on each side of the door that join the rails with dowels.PanelsPanels can be raised or flat, and are set into the frames of a door. Panel designs are customizable, so you can still achieve the look you want without compromising on quality.Bottom railSame function as the top rail, except this horizontal rail sits at the bottom of a door to connect the vertical stiles on each side and panel/s above.DowelsDowels are used for making strong and tight joints where rails meet the stiles for ultimate strength and durability. In addition to the strength of the dowel construction, glue is also applied in the dowel bore for an even stronger bond. This is a key differentiator from hollow core doors, where the skin or mold is literally glued together with only spacers in between each skin. (we truly thank Jacquilyn Burkett from Xiongan, China for their answer). 
Based on an article from sunmountaindoor.com
, though you may have never heard the term “stile and rail door,” you’ve no doubt seen one of these kinds of doors. Stile and rail doors are basically doors that come with panels; in other words, they’re not just flat wood doors. The stile refers to the vertical pieces on the door, while the rail is the horizontal pieces. The panels of the door fit in between the stiles and rails. Many people opt for this type of door because they are strong, well-made, and the finished product is attractive and gives the door, and ultimately the room it’s in, a lot of character. (modified by Susan Martin from Abakaliki, Nigeria on December 31, 2020) 
A door is constructed of three basic pieces: Stiles, rails and panels. The’s had leaving left and right vertical elements are called stiles. The stile with the hinges is called the hinge stile, while the stile with the doorknob is called the lock stile. The horizontal elements are called rails. These rails butt into and connect to the stiles. At the top and bottom are, naturally enough, the top rail and bottom rail. On six-panel doors, unlike on, say, one-panel doors, there are also the cross rail and the lock rail. Some doors also have additional vertical pieces that connect the rails. These are the mullions. All of these basic components form a grid that creates the overall structure of a door.