Speaker connectors do exactly as the name implies; they connect a passive speaker to an audio source, such as a power amplifier, or powered mixing console. The modern day industry standard speaker connector is the Speakon plug, developed by Neutrik AG, and is the most common connector used for the transfer of speaker signals between all modern pro audio equipment.
They come in variations, being 2-pin, 4-pin, or 8-pin. The reason for this is that speaker cables must often transfer multiple signals to one speaker, in order to power the more multiple parts of the speaker (horn and woofer).
A 2-pin speaker connector can transfer ONE powered speaker signal, while a 4-pin speaker connector can transfer TWO powered speaker signals, and an 8-pin speaker connector can transfer FOUR powered speaker signals.