Garlic scapes are the tops of wild garlic- The same wild garlic that grows everywhere, that makes your (my) lawn mowing experience smell like onions because you (I) mow the clumps of garlic instead of pulling them out. And the same wild garlic that colonizes farm fields. Farmers have gotten wise to this weed. They have discovered that the tops are a delicacy, and they have started to sell them at farmer's markets. When I see the garlic tops in the forgotten spaces in my yard, I know they are only a week or two behind at the market.
You want to harvest garlic tops when they look like the ones in the picture (there's a rain barrel behind them, in case you were curious). If you wait until the little balls form inside of the thin spade, the tops will be tough. You can eat the entire stem including the very top; they come in the markets in curled bunches. Last summer, I marinated the tops in teriyaki and threw them on the grill with a bunch of other vegetables at my Fourth of July picnic. A friend told me yesterday that she can not wait for them to come back in season so that she can do the same thing.