Frikadellen is a more formal term for "Fleischkloesschen" (meat balls). They are also known as "Bouletten" in Berlin. Frikadellen are made out of ground meat, usually half and half, with an egg and soaked bread bun added. They are fried in fat until the crust turns golden brown. You get them at fast food stalls, usually with ketchup or mustard.
"Bouletten" is a regional term for meatballs. The term is derived from French immigrants and understood in Berlin and Brandenburg. This is typical fast food and available at butchers, groceries and fast food stalls.