Odin is Samsung Flashing Tool that you required to flash official firmware as well as leaked firmware. ROM files flashable with Odin come with .tar extension. Custom ROMs are rarely flashable by Odin because they come with .zip extension that Odin does not recognize (it recognizes .tar files).
Custom kernels, however, are sometimes comes with .tar format (e.g. CF-Root kernels), so that they can be flashed by Odin. Usually, when your phone is new and running official firmware you most often cannot flash a custom ROM as it void warranty or because a Samsung phone often requires a custom recovery and root rights that are included in a custom kernel to be able to flash custom ROMs. That’s why Odin often comes in handy in rooting and flashing a custom firmware to your phone because you (often) can flash a custom kernel with it that already includes root and custom recovery and enables you to flash custom firmware (custom ROMs).
The latest version of Odin is version 3.12.3.