I have builded the U-boot for minnowboard max. I am seeing the files like uboot.rom uboot.bin etc.
what is the difference between uboot.rom and uboot.bin ? Which files I should flash to SPI NOR flash.
This is explained in doc/README.x86. In short, if you are going to be writing U-Boot to SPI NOR then you need to ensure that you have the correct binary blobs in the correct locations AND use BUILD_ROM=y so that u-boot.rom is generated as this is the file that is required on x86 to run on bare metal (rather than say as a coreboot payload).
Edit to address the comment: The file 'u-boot' is the ELF object that is the result of building all of the U-Boot sources and linking them. This includes all of the extra sections and information an ELF file can contain. This is also by and large not bootable. The u-boot.bin file is the ELF u-boot but passed via objcopy to strip out (by and large, see the Makefile for the various flags or build with V=1) everything except for text/data sections so that we have only what is required to boot. Then u-boot.rom is the combination of objects and formatting that the x86 architecture requires in order to execute and run an image. Building with V=1 will show all of the details here.