DFU stands for Default Firmware Update.Recovery mode and DFU (Device Firmware Update) iPhone allow us to restore the device firmware using iTunes. Visually we can differentiate from each other because in the first (recovery mode) is displayed on the screen of the iPhone the iTunes logo while the second (DFU mode) screen remains in black.
When iTunes does not recognize the device or say that it is in recovery mode.
When you see the Apple logo on the screen for several minutes and the progress bar does not appear.
When you see the Connect to iTunes screen.