The case holds all the parts together. Its basically the housing of your keyboard.
Image via KBD fans
Plastic
Plastic cases are the cheapest to make. Plastic cases are lighter and good for those who are on a budget. They tend to sound rattily, and are less rigid then other metal cases.
Image via KBD fans
Aluminium
Sturdier, stronger and has a nice weight to them. I really like Aluminum cases. They make your keyboard feel expensive.
Image via KBD fans
Acrylic
Acrylic cases are light and allow lighting effects to shine through. Really nice to look at especially if your PCB has LEDs.
The most common mounting style is the tray mount. It is where the PCB is fixed to the case by screws. The mounting style will determine how your keyboard sounds and feel when you type on it. If you have a bit more cash to spend I would recommend looking out for kits with the gasket mount option. The are known to sound awesome and is a dream to type on. For an in depth breakdown on the myriad of mounting styles I highly recommend you check out Thomas Bart cheat sheet on mounting styles.
Same as the plate and PCB they usually are sold together in keyboard groupbuys.
Links:
Thocstock - For In-stock parts
Mechgroupbuys - They track keyboards group buys