The USB serial interface on most Arduino boards can only be used as a serial port, i.e. to communicate with terminals and applications specifically designed for it. On the UNO, however, the USB controller can be reflashed to work as a HID device, though that is an advanced effort and should probably not be done by beginners. The upcoming Leonardo board is native USB, so that would be usable for things like USB input devices. Using a microcontroller as storage device is a very bad idea, as there is hardly enough space for a single text file unless you add external memory.