Introduction to Shellcode: How to exploit buffer overflows. A very thorough and well written paper on how it all works. A good read for anyone curious and even those who already think they know it all. It includes step by step examples from vulnerability discovery to a finished exploit. The paper focuses on x86 Intel syntax assembly under Linux.
You can download it from the following link: https://packetstormsecurity.com/files/download/34622/intro_to_shellcoding.pdf
Source: https://packetstormsecurity.com/files/34622/intro_to_shellcoding.pdf.html