Text consists of intermingled character data and markup. In XML documents some characters are used for start-tags, end-tags, entity references, XML declarations and others. This means that if you'll want to use some of those characters in a text writing you'll get errors.
If you want to use these characters in XML documents then you'll need to replace/escape them using numeric character references or using strings.
String replaces:
' '
< <
> >
& &
