I saw a code snippet the other day that converts a Boolean value to the corresponding \"Yes\"/\"No\" value:
CDbl(True).ToString(\"Yes;Yes;No\")
Steve-X has the best documentation for String.Format I've seen so far: Steve-X ToString()
I know you asked for "ToString", but ToString is dependent to the implementation you are calling (i.e. DateTime.ToString(), decimal.ToString...etc).
If you are really interested in how it works bust open reflector and peruse the code.