Banana plugs are 4mm connectors widely used in Europe for connecting speakers to amplifiers and receivers. In the US spade plugs are often used instead, although banana plugs are generally considered superior as they offer a bigger area of contact and are easier to plug in.
Using banana plugs have a number of advantages over using just the bare wire to terminate your speaker cable. They are a lot easier to plug in and out and cannot become frayed like bare wires can. They also give an airtight seal meaning the speaker cable does not become oxidised, leading to degradation in performance.
Unfortunately, banana plugs are now illegal in Europe. 4mm just happens to be the same size as most European power sockets (though not British ones) and the bureaucrats decided that the risk of someone plugging their speaker cable into the power socket was too great. Many manufacturers still provide banana plug connections in disguise, usually meaning the end user has to remove a small piece of plastic to use them.
Prices for banana plugs range from around £1 (~$1.5) to £25 (~$37.50) each.