The quality of prefab cabinets is as varied as the cabinets you purchase pre-assembled. At the low end are pieces made from inexpensive pressed wood faced with laminate. Average quality cabinets are made from medium-density fiberboard, or MDF, which is also a manufactured wood product but is smoother than pressboard and has no grain, so drilling and sawing don't damage the surface. High-end prefab cabinets, like their pre-assembled counterparts, are made from hardwood or a combination of hardwood and other wood covered with wood veneer. You can purchase wood cabinets finished or ready to stain or paint.
Prefab cabinets are flat-pack items; the disassembled pieces are stacked and packaged in rectangular boxes that are just inches deep. This makes the cabinets much easier and much less expensive to ship from manufacturer to retailer and from retailer to consumer. Flat-pack items are easier for stores to stock, too. This can save you money by eliminating shipping fees in many cases, because you can find a large selection of prefab cabinets in stock at local retailers and transport your purchase home using your own vehicle.
Prefab cabinets come in pieces--base, top, sides, shelves, backing, doors (if applicable), and finishing touches like hinges, drill-hole covers and end caps. All the hardware you need for assembly is typically included also. The manufacturer precision-cuts each piece and drills holes into the exact spots where you'll insert screws to assemble your cabinet, so the pieces should fit together perfectly. Step-by-step instructions guide you through assembly.
Give yourself ample space to assemble the cabinet. Remove other furniture from the room, if necessary, so you can spread out and maneuver around the cabinet sections as you work. You'll find instructions in the box that list the cabinet parts and the assembly hardware the manufacturer has included, as well as the tools or other supplies you'll need. Verify that you have each item before you begin. Double check each cabinet section before you secure it to make sure you're attaching it to the right side and that it's facing in the right direction. Finally, move the cabinet as little as possible until you've secured the backing. Prefab cabinet backing is often just a thin sheet of manufactured wood or cardboard, but it adds a good bit of stability to the furniture.