Hot and sour soup is often spicy (though recipes vary) and always mildly sour due to the inclusion of vinegar. It usually has bean sprouts, tofu, and mushrooms, and is generally my preference when at a Chinese restaurant.

If you'd like to try making the soup, here's a recipe from Epicurious that got good reviews.

Hot and Sour Soup with Ginger

4 cups canned low-salt chicken broth
1 1/2 ounces dried shiitake mushrooms
5 tablespoons rice vinegar
2 tablespoons cornstarch

1 1/2 tablespoons oriental sesame oil
3 tablespoons minced peeled fresh ginger
1 10.5-ounce package extra-fine or firm tofu, cut into 1/4-inch dice
1 ounce bean thread noodles or angel hair pasta, broken in half
1 tablespoon soy sauce
1/2 teaspoon dried crushed red pepper
Pinch of sugar

Combine broth and mushrooms in bowl. Let stand until mushrooms soften, about 20 minutes. Remove mushroom from broth and squeeze dry over bowl with broth; reserve broth. Discard mushroom stems; thinly slice caps. Combine vinegar and cornstarch in small bowl; stir to blend.

Heat oil in large saucepan over high heat. Add ginger; sauté 30 seconds. Pour in reserved broth, leaving behind any sediment. Bring to boil. Add tofu, noodles, soy sauce, red pepper, sugar and mushrooms. Reduce heat to medium-low, cover and simmer 5 minutes. Add cornstarch mixture; stir until slightly thickened, about 1 minute.

It's a fairly normal soup in that it has a meat base with various veggies. The hot and sour name comes from the hot pepper and vinegar that's in it. Makes it tasty, good stuff... I'd recommend trying it out.