Lightning is a proprietary I/O interface designed by Apple for its mobile devices, such as the iPhone, iPad, and iPod. It was first introduced in September, 2012, with the iPhone 5 and new iPod models. It was later added to iPads, beginning with the 4th generation iPad and the first generation iPad mini. The Lightning interface replaced the the previous "dock connector," which Apple products used since 2003.
The Lightning connector has eight pins and is about one third the size of the 30-pin dock connector it supersedes. Instead of having mobile latches on the sides, the Lightning connector has small divots on each side that allow it to snap into place. Even without magnets or clips, the Lightning connection is designed to be strong enough to hold a device upside down by the cable without the cable detaching.
Unlike most other I/O interfaces, such as USB, Firewire, and the previous dock connector, the Lightning connector is reversible. The connection is fully symmetrical and the connector can be inserted either way into a Lightning port. This means it is impossible to insert the cable upside down, which makes it easier to plug in and reduces wear and tear on the interface. The order of the pins are recognized dynamically by the device when the connection is made, allowing power and data to flow through the correct channels.
The name "Lightning" is correlated with Thunderbolt, another I/O port Apple began using around the same time the Lightning interface was introduced. However, unlike Thunderbolt, Lightning is proprietary and is only used in Apple products. While most mobile devices (such as Android phones) have standard mini-USB or micro-USB ports, if you have an Apple device, you will need a Lightning cable to charge it and transfer data.