A lot of folks think that the 6 panel oak interior doors they can buy down at the local lumber chain are solid wood doors. Chances are they are probably veneer doors. Veneer doors with raised or flat panels actually have a very thin layer of real wood glued to a man made core of particle board or medium density fiberboard aka MDF. Veneer doors are just as heavy as solid wood doors and most folks can't tell the difference unless they know exactly what they are looking for. You can usually see the man made core by looking at the top or bottom edge of the door or inside the hole for the doorknob.
Some veneer doors also have glued up wood stiles and rails and only have the man made cores in the panels. If this is the case then the only way to determine if the door is a veneer door is to look closely at the veneer thickness on the ends of the doors. You may need a magnifying glass. The wood on veneer doors is 1/16" or less. Higher quality veneer doors will have veneer thicknesses of between 1/32" and 1/16". Cheaper veneer doors may only have a 1/64" or less veneer thickness. Usually the veneer on the panels is a little thinner than the veneer on the stiles and rails especially on raised panel doors because the veneer actually has to be vacuum pressed to conform to the panel configuration.
Veneer doors are not inferior, they are just different. They have some advantages and some disadvantages over solid wood doors. Advantages include less expansion and contraction and fewer seams in the panels. Veneers are often higher quality and more consistent in color than solid wood. Veneer doors make better use of high quality wood resources and are often more economical than solid wood doors. However, care must be taken not to sand through the thin veneers. The hardness of the wood is not as important to wearability and impact resistance because the door will only be as hard as the substrate underneath the veneer.
So when deciding on interior doors for your home you can now compare quality and construction.