simple or complexContent?

Is there a simple description somewhere of what is meant by simpleContent 
or complexContent?

I am defining a new element as a complexType that is based upon another 
type. Basically I'm adding an attribute to  another complexType comprised 
of string content and another attribute. Should this be simpleContent or 
complexContent inside the new complexType?

XML Spy is reporting a problem with one instance of this construct. I have 
6 other similar elements and definitions that it doesn't complain about. In 
trying to read the Schema spec rules on this subject you need a road map.

..dan