Where were you in the 90s (let alone the current century)? That’s when arugula pretty much became our national salad green. Actually, it was popular in Italy and France for ages, but caught on in the US and Britain somewhat more recently. Perhaps you know if by some of its other names - rugola, rucola, roquette, garden rocket, Mediterranean rocket, salad rocket, Roman rocket, or Italian cress (the rockets being a corruption of the French roquette).
Arugula is a spicy little leaf, which some describe as bitter and others characterize as having a "peppery-mustardy" flavor. Because it is so potent on its own, it is often mixed with milder greens to produce a nice balanced salad. It can also be sautéed in olive oil. You can substitute most any green for arugula, but the closest matches are Belgian endive, escarole, and dandelion greens.