Spring or Recoil mechanism that draws the tape back into the plastic or metal housing.

You can actually replace tape in a tape measure by threading out the entire metal tape, unclipping it from the recoil mechanism and attaching the new one, but often the cost of a new tape is more expensive than buying an entirely new measuring tape.

Outer housing, locking pin and screws that hold it all together

The frog is the metal tab at the end of the tape.

It is suppose to slide back and forth! It should move the thickness of the metal end (approximately 1mm), to compensate for this thickness when taking internal measurements, say inside a drawer.

The best tapes have 3 rivets anchoring it to the tape... two that actually hold the floating tap and a third rivet that holds a reinforcing plate onto the back. If it only has one DON'T BUY IT.. it will only last a short time.

Belt Clip... the part I expect all you women woodworkers will figure out how to use :) .. the men already know!

Here's my old beat-up Lufkin tape measure. I really love the rubber grip around the front and the firm locking pin in the front that is imbedded in the body so it doesn't break when I drop it.. some really stick out and last about a week before they're history.

Test it to see how firmly the locking pin holds the tape in position as well.