Hi guys

I have galaxy S2 and I am new to the android world
according to the storage option in the setting it shows that I have total space 1.84 GB in SD card(2 GB in actual), 11.50 GB in the USB storage and 1.67GB available space in device memory, my question is what is the what is the difference between device memory and USB storage and how can I use the available space in the USB storage