Let's break the word shellcode into shell and code. In simple terms, a shellcode is a code that is designed to give a shell access of the target system. Practically, a shellcode can do lot more than just giving shell access. It all depends on what actions are defined in the shellcode. For executing client-side attacks, we need to choose the precise shellcode that will be part of our payload. Let's assume, there's a certain vulnerability in the target system, the attacker can write a shellcode to exploit that vulnerability. A shell code is a typically hex encoded data and may look like this:
""\x31\xc0\x31\xdb\x31\xc9\x31\xd2" "\x51\x68\x6c\x6c\x20\x20\x68\x33" "\x32\x2e\x64\x68\x75\x73\x65\x72" "\x89\xe1\xbb\x7b\x1d\x80\x7c\x51" ...
