Minced and reformed beef in a vaguely disc-shaped slab, cooked, and placed inside a bread roll, possibly along with various vegetables and sauces. Usually the meat is sourced from cheap off cuts and has a higher than average fat content. That being said, there is something distinctly refreshing about biting into a great greasy burger once every so often.
*The best burgers in existence are thick, juicy, and served on a perfectly baked white bread roll. None of this industrialised chain crap!