OTG is an abbreviation for “On-The-Go”. It is a specification that allows devices such as smartphones to act as a host, allowing other USB peripheral devices like a USB disk-on-key, mouse, keyboard or camera to be attached to them (this is why OTG capability is sometimes referred to as “host mode”).
Unlike conventional USB connectivity, in USB-OTG connections the host (in our case the Smartphone) will provide 5V power feed to the peripheral device (in our case the Therm-App™ device). It will also control and read the raw data from it.
The USB-OTG standard makes use of a special wiring trick inside the USB connector in order to signal the Smartphone that it should become a host. This is why Therm-App only works with USB-OTG cables. For more info see: http://en.wikipedia.org/wiki/USB_On-The-Go or http://www.usb.org/developers/onthego/