u-boot.bin is the binary compiled U-Boot bootloader.u-boot.img contains u-boot.bin along with an additional header to be used by the boot ROM to determine how and where to load and execute U-Boot.
The way in which these files are deployed can depend upon the nature of your device, its boot ROM and where the files are loaded from.
Boot ROMs are generally provided by the SoC/CPU vendor. These days, many boot ROMs are capable of loading
u-boot.img, reading the file's header, loading u-boot.bin into memory and finally executing it. Some boot ROMs are complex enough to load u-boot.bin directly or even the OS kernel. While others may load an intermediate bootloader (MLO/X-Loader) first which then takes responsibility for loading U-Boot as the secondary bootloader once external memory is initialized.
This image depicts the latter case as implemented by some TI OMAP processors: 
This boot process is reduced by some devices by moving many of the X-Loader tasks into U-Boot and placing boot parameters (such as memory addresses) into the header of
u-boot.imgavoiding the need for an intermediate bootloader.
System Boot process:
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