Shellcode is machine code that when executed spawns a shell, sometimes. 
Shellcode cannot have any null's in it because it is (usually) treated as a C 
string and a null will stop the reading of the string, as it is the string 
delimiter. Not all "shellcode" spawns a shell, this has become a more generic 
name for a bit of position independant machine readable code that can be directly 
executed by the cpu. Shellcode must always be position independant - you cannot 
access any values through static addresses, as these address will not be static 
in the program that is executing your shellcode - environment variables are the 
execption to this rule. Remember to always use the smallest part of a register
possible to avoid null's, and xor is your friend.