If you live in an area with a big Asian community then you've probably seen little old Asian ladies hawking produce on the side of the road. They stand out in the hot sun selling produce at very cheap prices, and they are there day after day. Right now the Korean ladies are selling stacks of perilla leaves, though if you go to any Korean grocery store, you'll see them being sold as sesame leaves. I don't understand why they refer to perilla leaves as sesame leaves, but they do.
Perilla is actually the term for a number of different species of plants in the mint family. The Japanese use shiso, which is smaller and mintier than the broad, rounded perilla leaves favored by the Koreans.
The flavor of perilla, grassy with notes of anise or licorice, is pleasing like any other herb. The real advantage is their size. You can do more with the bigger leaves than just chop them up and use them as a garnish (though doing so is perfectly acceptable and delicious).
Perilla leaves can be stir-fried with garlic and veggies, deep-fried in a batter of flour and eggs, pickled or marinated, or used as wrappers. (Or any combination of these things, like wrapping something in perilla then battering it and deep-frying in oil.) If I have extra perilla leaves on hand, I like to use them as I would lettuce, wrapping the leaves around rice and bits of seasoned fish or pieces of grilled meat.
One of my favorite preparations for Korean perilla leaves is to marinate the leaves in a mixture of soy sauce, sesame oil, vinegar, garlic, and lots of Korean chili powder. The result is a mildly pickled, spicy side dish that takes no effort and is as addicting as kimchi.