Only the x86 instruction set is mostly described here.
Basics
What is Assembly Language
Assembly language is a low-level programming language for a computer or other programmable device.
What is the difference between the Assembly language and high-level programming languages
Each assembly language is specific for a given computer architecture (instruction set).
High level programming language are mostly portable across multiple systems.
How is the source code of an high-level programming language converted to the executable machine code
Via Compiler
How is the source code of an assembly language converted to the executable machine code
Via an Assembler
List examples of some assemblers
NASM, MASM.
External links
https://www.tutorialspoint.com/assembly_programming/index.htm