Buttercream is a term that has been applied to a variety of frostings, many have butter, some don't, and the steps to make them can differ. You may recognize one or two of these.
Decorator's Buttercream is a white, white frosting acheived by combining shortening, powdered sugar, flavorings and usually hot water and mixing until smooth on a mixer with a paddle attachment.
American Buttercream uses the same method, but replaces the shortening and water with butter and milk. These first two frostings are generally fairly sweet and heavier on the tongue than the following meringue based frostings.
Swiss Meringue Buttercream utilizes a different method. Egg whites and granulated sugar are combined over heat until the sugar dissolves. This combo is then whipped at high speed to a light meringue(hence the Meringue in the name). Softened butter is whipped in at the end to create a light, smooth frosting.
Italian Meringue Buttercream uses the same ingredients as Swiss, but cooks the sugar to 240 degrees before pouring, Carefully!, over the whipping egg whites. Like the Swiss meringue version, softened butter is whipped in at the end. This frosting is slighty stronger than the Swiss meringe. (At least in my opinion).
French Meringue Buttercream follows the Italian method, but uses whole eggs instead of only the whites, for a richer taste and color.
5 frostings, 3 methods, 1 name. Now you know what is in a buttercream.