An element's content is defined by combining a construct called a 'particle':
'sequence' element - a sequence of children elements.
'choice' element - a choice of children elements.
'element' element - a specification of a child element.
'all' element - any ordering of child elements.
'any' element - any elements from a particular namespace.
This lecture will cover 'sequence', 'choice', and 'element'.
Keep in mind that 'any' and 'element' cannot be used as a direct child of 'complexType'. :(