Basic Encryption Question

I have been looking at ways of encrypting querystrings and I have found a few good examples.. but, I just want some clarification on what is most often referred to as the EncryptionKey and the characters that are in that key....


Is there basically an unlimited number of alphanumeric combinations that could be used in the following code as the encryption key, is there anything that someone should be CAREFUL about including in that key?


thanks...


using System.Security.Cryptography;
private static string EncryptionKey = "!#$a54?3";
/// other exam ...