Difference between uboot - spl and previous u-boot versions

We can also call SPL as x-loader and UBL etc.,
Its small boot loader which will fit into internal RAM to enable the external RAM (DDR,mDDR, SDRAM etc.,) for running the u-boot on it.

To know more, please refer to the following links.

processors.wiki.ti.com/.../Boot_Sequence
omappedia.org/.../Bootloader_Project