A corned beef sandwich is a sandwich filled with corned beef. The salt beef style corned beef sandwiches are traditionally served with mustard and a pickle.
In the UK, pickle is a common addition to a corned beef sandwich. ("Corned beef" in the UK refers to what is called bully beef elsewhere.)
Another variant more common in the United States has sauerkraut, known as a Reuben sandwich.
A contraband corned beef sandwich on rye brought aboard the Gemini 3 spacecraft by John Young resulted in a minor controversy, for the risk posed to the craft and crew by floating crumbs and lingering odors.
Media related to Corned beef sandwiches at Wikimedia Commons