Axle Type. Either (1) Quick Release or (2) Thru Axle or (3)Bolt on. You must specify the type of axle that fits your frame.
Quick Release (QR) is the most common on road bikes and touring bikes. The axle is hollow and a thin rod slides through it and a toggle tightens the nut. The most common diameter was 9 or 10mm
These attach to the frame by threading into the fork or dropout, rather than sliding into a slot. They are usually thicker.
Really cheap wheels may come with a threaded axle, held on by two nuts.