USB refers to a type of hardware for transferring files and power. It is not a description of a particular file format (like JPEG, PDF, DOC). The device is designed to let you transfer certain files through a USB connection, say from your camera or a flash drive.
Note from the comments, though, that this device will look for only one type of file, a DCIM file, which is a file that most digital cameras create on a disc and store photos in. So if you want to transfer photos from something like a flash drive, you will have to put them into a DCIM file first. See comments about this.