what's difference between "relocatable link" and "final link"?

["Huang, Chunhao" <Chunhao dot Huang at sony dot com dot cn>]

Greetings,

This is chunhao.

When I study the BFD library, I meet "relocatable link" and
"final link" several times, but I can NOT find out the difference
between them. My rough comprehension is that "relocatable link"
done by assembler, which is something like parsing the local
symbols in sections. while "final link" done by linker.

But I find such sentence in bfdint.text:
"Make sure you clearly understand the operations the linker must
perform during a relocatable link and during a final link." 
Which means that both "relocatable link" and "final link" are
performed by linker. So my orignal comprehension is not correct.

Is there any guy who can provide me the difference between them?
If you can provide me some simple examples in ARM or x86, that's
great.

Thanks a lot.
Best Regards

Chunhao