I love the pungent leaves of the plant that I call "arugula." Nothing spices up a salad like arugula, but there appears to be a great deal of confusion surrounding this arugula (Latin name,
Eruca sativa). It has many common names (besides arugula, referred to as rocket, rocket salad, roquette, rucola, and garden arugula, among others, with arugula and rucola coming from the Italian and rocket and variants coming from the French roquette) and it's often confused with (or wrongly thought to be the same as)
Diplotaxis tenuifolia, which likewise has a number of common names; I've seen it called wild arugula, perennial arugula, perennial wall arugula, and, in plastic bags at the supermarket "baby arugula." While both are in the mustard family (Brassicaceae), they are not in the same genus and therefore not closely related.
I know the two are distinct simply because the Latin names differentiate them, which is the whole point of Latin names – to distinguish between plants and animals with overlapping common names and different names in different languages around the world, but I also know how different they are because I grow both types in my garden. Eruca sativa, which comes up from seed quicker than just about any other plant I've ever sown, is an annual that grows quickly, producing flat, notched leaves typically about three to six inches long. It is fairly quick to bolt in the summer. Its flowers, on stems up to about 18 inches tall, are typical mustard family flowers with four petals in a bilaterally symmetrical arrangement, pale cream colored with fine, dark, purplish veining (see photos below).

Diplotaxis tenuifolia looks altogether different. The leaves are shorter, narrower, and more heavily if not more deeply notched. It forms mounds of dainty foliage, growing up to about 24 inches high or as much as about 36 inches including the long, thin flower stalks. The flowers are again typical mustard family flowers with four petals, but the flowers are a plain, unveined yellow (see photos). Finally, Diplotaxis tenuifolia is a perennial.
What the two plants have in common is the presence of chemicals in the leaves of glucosinolates that break down into isothiocyanates, which as a group (if I understand correctly) are referred to as "mustard oils." These are the compounds responsible for the sharp and bitter flavors we know from mustard, radishes, watercress, capers, and others – including arugula, or rucola, or rocket, or roquette, or perennial arugula. The two plants smell and taste similar, so they share names, despite their taxonomical distance.
I find it frustrating that there seems to be no agreement about the common names. I've seen almost all of the names listed above for Eruca sativa used for both plants. I've even seen Japanese mizuna (which is another mustard family plant altogether,
Brassica rapa var. nipposinica or
Brassica rapa var. japonica) erroneously referred to as "arugula." While the Latin names clear things up and I'm perhaps a nerd, I'm not nerdy enough to ask for
Diplotaxis at the supermarket. It's bad enough when I pronounce shiitake properly and they have no idea what I'm talking about.