Beer batter is, appropriately enough, deep-frying batter made with beer as the main liquid ingredient. Using beer instead of water gives the fried breading a unique flavor, and a more tender, consistent texture. Many people customize the recipe, using different beers and spices to enhance the flavor of whatever food is being fried (eg: lager for fish, amber for chicken, etc.). The recipes can get involved, especially where the spices are concerned, but the basic required ingredients are: flour, oil, eggs, and beer. I'm including a couple of different beer batter recipes below, for contrast.

Basic beer batter
1 egg
1 c beer
2 tb vegetable oil
1 1/4 c flour
1 ts black pepper
1 ts salt
1/2 ts cayenne pepper

Beat the egg in a medium sized bowl, and add the beer, oil, and spices. Mix well. Gradually add the flour to the mixture, beating until smooth. Let the finished mixture rest for 30 minutes before using it. Refrigerate any remaining batter.

Beer batter for white fish
2/3 pound flour
3 ounces baking powder
5/8 teaspoon white pepper
1 teaspoon granulated garlic
1/2 teaspoon ginger
1/2 teaspoon salt
2 3/8 cups lager beer
4 whole eggs
1 1/2 tablespoons sherry

Combine dry ingredients, and blend well. Combine liquid ingredients in a separate container, and mix well. Gradually add dry mixture to wet mixture, and beat until smooth. Let the mixture sit for 30 minutes before use.