What does ORG Assembly Instruction do?

can anyone give me a comprehensive description about ORG directive?
When and why is it used in assembly written applications?

Using Nasm on x86 or AMD64.

Solution 1:

ORG is used to set the assembler location counter. This may or may not translate to a load address at link time. It can be used to define absolute addresses, e.g. when defining something like interrupt vectors which may need to be at a fixed address, or it can be used to introduce padding or generate a specific alignment for the following code.

Solution 2:

ORG is merely an indication on where to put the next piece of code/data, related to the current segment.

It is of no use to use it for fixed addresses, for the eventual address depends on the segment which is not known at assembly time.