Quick Release (QR) wheels:

Refers to how the wheels fasten into the frame and forks. A quick release wheel can be removed from the bike without using a spanner, since the axles are replacesd with shorter hollow ones, though which a lever is passed. This lever operates a cam tightener which clamps the wheel into position. Being able to remove your wheels without a sanner means you can more easily repair punctures or put your bike in the boot of your car etc. Without quick realease, you would need a spanner to tighten and untighten the wheel nuts.