USB On-The-Go, often abbreviated to USB OTG or just OTG, is a specification first used in late 2001 that allows USB devices, such as digital audio players or mobile phones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, mice or keyboards, to be attached to them. Use of USB OTG allows those devices to switch back and forth between the roles of host and client. For instance, a mobile phone may read from removable media as the host device, but present itself as a USB Mass Storage Device when connected to a host computer.
In other words, USB On-The-Go introduces the concept of a device performing both master and slave roles – whenever two USB devices are connected and one of them is a USB On-The-Go device, they establish a communication link. The device controlling the link is called the master or host, while the other is called the slave or peripheral.
Source from Wikipedia.