Angel food cake is a cake made with a lot of egg whites and usually no shortening or leavening agent. It has the consistency of a sponge cake.
From WhatsCookingAmerica.net comes this piece of information about Angel Food Cake:
CHOCOLATE ANGEL FOOD CAKE
Ingredients
1 1/2 cups sugar

1/4 cup cocoa powder
1 tsp baking soda
1/2 tsp salt
1 ounce dark chocolate, grated (about 1/2 cup)
12 large egg whites
1 tsp cream of tartar
1 tsp pure vanilla extract
1/4 tsp pure almond extract
Directions
Preheat oven to 350 degrees. Place oven rack in center.
Sift together onto piece of parchment paper: 3/4 cup sugar, flour, cocoa powder, baking soda, and salt. Add grated chocolate; set aside.
In bowl of electric mixer fitted with whisk attachment, beat egg whites and cream of tartar until soft peaks form. With machine running, add remaining 3/4 cup sugar in slow steady stream, beating until fully incorporated and stiff glossy peaks form. Add vanilla and almond extracts; beat to combine.
Remove from mixer. Gradually add flour mixture, gently but thoroughly, folding into egg-white mixture until fully combined. Pour into nonstick angel food cake pan. Tap pan on counter to remove air bubbles. Bake until cake springs back when depressed with finger, 30 to 35 minutes.
Cool, inverted, 1 hour before removing pan.