Cumin is probably the most prominent spice in falafel. I have never had it made entirely out of bulgur, but the recipe I use says you can add some of that instead of flour to help bind it. I've never used it myself though, I just use regular AP flour. Recipes for falafel are as subjective as those for minestrone or chicken & biscuits; everyone is sure theirs is the only real one out there. =) Here is the recipe I have used for ages. The cilantro and parsley can both be omitted, or you can use just one or the other. I use whatever I happen to have in the fridge.
Falafel
Epicurious / Yield: About 20 balls
1 cup dried chickpeas
1/2 large onion, roughly chopped (about 1 cup)
2 tablespoons finely chopped fresh parsley
2 tablespoons finely chopped fresh cilantro
1 teaspoon salt
1/2-1 teaspoon dried hot red pepper
4 cloves of garlic
1 teaspoon cumin
1 teaspoon baking powder
4-6 tablespoons flour or bulgur
Soybean or vegetable oil for frying
Put the chickpeas in a large bowl and add enough cold water to cover them by at least 2 inches. Let soak overnight, then drain. Or use canned chickpeas, drained.
Place the drained, uncooked chickpeas and the onions in the bowl of a food processor fitted with a steel blade. Add the parsley, cilantro, salt, hot pepper, garlic, and cumin. Process until blended but not pureed.
Sprinkle in the baking powder and 4 tablespoons of the flour, and pulse. You want to add enough bulgur or flour so that the dough forms a small ball and no longer sticks to your hands. Turn into a bowl and refrigerate, covered, for several hours.
Form the chickpea mixture into balls about the size of walnuts, or use a falafel scoop, available in Middle-Eastern markets.
Heat 3 inches of oil to 375 degrees in a deep pot or wok and fry 1 ball to test. If it falls apart, add a little flour. Then fry about 6 balls at once for a few minutes on each side, or until golden brown.
(Instead of frying mine in oil, I drizzle oil in the bottom of a baking dish and then preheat it, along with my oven, to about 375. Then I let them bake for about 20 - 25 minutes.)
EDIT: Just wanted to add that I've substituted a can of garbanzo beans for the soaked dried ones with equal success.