If you need screws for your computer or are looking to buy extras, it's not always clear what types of screws you're dealing with. Thankfully, there are only three main types, and each is easily distinguished visually.
The "6-32" Screw
This one usually has a hexagonal head. That means that the top of the screw, where the screwdriver goes, has six sides on it. Any #2-sized Philips screwdriver will fit into this head. Flathead screwdrivers can be used in a pinch, but this option requires more downward pressure to ensure insertion.
M3 Screws
This is a metrically measured screw, and smaller than the 6-32 screw. It is often used to attach a hard drive or optical drive. While 6-32 screws may fit in M3 holes, they will usually not fit into the screw holes of the drive itself. You should use M3 screws to avoid damaging the drive in this situation.
Brass "Standoff"
These are used to support the motherboard slightly above its tray. If the motherboard touches the case, it can cause an electrical short, so the board is secured with brass since that type of metal does not conduct electricity. These screws are secured from the other side with 6-32 or M3 screws that fit in pre-drilled holes on the motherboard.
Variants
You may see M3 screws with round, flat heads. These are distinguished from 6-32 because the rod (the threaded length of the screw) has a smaller diameter. Some have "wings" for tightening and loosening by hand. There is also the larger M4 and 8-32, but these are rarer.
Fan Bolts
Case fans are sometimes secured with long bolts instead of screws. These bolts thread from one corner of the fan through to the outside of the case and are secured with washers and hexagonal nuts. Sometimes the washers are rubberized to reduce vibrational noise.
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