A technology that allows non-computer devices (such as phones) to connect directly to peripherals (such as mice, keyboards, and memory sticks) via USB cable.
In a traditional USB setup, all devices are either hosts or peripherals. Computers are generally hosts, and everything else - including phones - are peripherals. This allows a phone to be connected to a computer for transferring files and sharing an Internet connection, for example. In any traditional USB setup, a host (computer) is required, and therefore a phone cannot be connected directly to a keyboard. USB OTG solves this problem by letting a phone play the role of either host or peripheral, depending on what it's connected to at the moment.
Not all phones with USB support USB OTG. Furthermore, a phone with USB OTG may only support certain types of peripherals.