The vacuum bag press is used to form a material, laminated or infused with glue, over a pre-constructed mold called a plug. The creation of a vacuum inside a sturdy bag surrounding both components causes atmospheric pressure to clamp the material to the plug while the glue dries/cures, so that it takes its form.
The most basic way to do this is to create a molded plywood, made up of thin wooden veneers glued together over the plug. As the glue dries, the veneers lock each other into place, so the plywood maintains the form of the plug after it is released from the bag. The drawback of this method is that the veneer can only flex in one direction, so it cannot be molded into a complex form.
If a complex form is desired, a second method, consisting of a woven substrate such as burlap, kevlar, carbon fiber, or fiberglass cloth, which is impregnated with epoxy (polyester resin can technically be used but it is extremely stinky, so it is forbidden in the SADR). The woven material is flexible in all directions so it can conform to very complex shapes, but the process is much messier and involves a number of additional materials such a bleeder fabric, release film, and screening.
A third method, similar to the second, involves packing the plug with fine, loose fibers, which are then impregnated with epoxy and bagged similarly to the second method. This allows for the building up of the laminated surface for added strength in certain areas or for aesthetic reasons.
Here is an exhaustive pamphlet on vacuum bagging:
And a comprehensive website: