OTG is short for On The Go and USB OTG is a standard that enables devices to talk to one another. For example, instead of connecting to a PC to transfer files, USB OTG enables devices to connect directly and swap stuff; instead of sending your photos to your printer via your PC, you can connect directly do it, and so on.
That's not all USB OTG can do, though. By enabling your device to act as a USB host – that is, to be the boss of other devices that you connect to it – you can add additional hardware, such as storage, keyboards or even musical instruments.
If your phone is one of the ones listed below, or a more recent model, then it'll almost certainly support USB OTG.

Sony Xperia Z, ZL, Z Ultra, Z1
Samsung Galaxy S2, S3, S4, Note 1, Note 2, Note 3
LG Optimus G, G2
HTC X, One, One mini, One max
Google Nexus 5


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