Thanks to A Pinch Of for the answer:
These small green spheres are the unopened flower bud of a Mediterranean bush, Capparis spinosa. Closely related to the cabbage family, the shrubby plant resembles a rose bush. Each bud is picked by hand in the early morning hours before it can open. Capers are harvested daily from May to July. They have no great taste appeal when eaten fresh but after pickling they take on a pungent flavor.
After drying the buds in the sun, they are pickled in a strong vinegar brine. Most are bottled in this brine but they may also be packed in salt. This salt-pack method is far superior for maintaining quality and flavor. In her book, From Julia Child's Kitchen, Mrs. Child suggests replacing half of the brine in a jar of capers with vermouth to improve their flavor.
Although they are grown throughout the Mediterranean as well as parts of Africa and Asia, the finest capers are said to be the tiny nonpareille (meaning 'without peer') that come from southern France. Capers range in size from this especially small variety to much larger ones from Italy. Morocco is the largest commercial producer today.
A new variety of capers from Spain is emerging on the market as "caperberries." The smaller the better has long been a mantra with capers but these caperberries are the size of olives. Packed with the stem intact, they are an elegant addition to a buffet with roasted fowl or poached fish.
ps. Click on the capers for one of my favorite caper inclusive recipes.