The Arduino has rows of connectors along both sides that are used to connect to electronic devices and plug-in 'shields' that allow the Arduino to do more.

However, the Arduino also has a single LED that you can control from your sketches. This LED is built onto the Arduino board and is often referred to as the 'L' LED as this is how it is labelled on the board.

The position of this LED is circled in red on the pictures of the Arduino Uno and Leonardo below.