XML predefines the following internal entities:
& gives you the ampersand character (&),
which cannot be used because it is used for introducing
internal entities.
< gives you the "less than" character (<),
which otherwise introduces tags.
> gives you the "greater than" character (>),
which otherwise termiantes tags.
" gives you the double quote (").
' gives you the single quote (apostrophy, ').
These two are especially useful when you have to use quotes in the value of an
attribute.
&#xxx;, which xxx being a decimal number,
gives you a 16-bit ISO/IEC 10646 character. This is also called a "character reference".
You can define your own internal entities
in the document header.