assembly language

A low-level programming language in which each program statement must correspond to a single machine language instruction that the processor can execute.

Assembly languages are specific to a given microprocessor and, as such, are not portable; programs written for one type of processor must be rewritten before they can be used on another type of processor.

You use assembly language for two reasons:

To wring as much performance out of the processor as possible
To gain access to specific characteristics of the hardware that might not be possible from a higher-level language