Convierte caracteres especiales en entidades XML seguras.
En XML, ciertos caracteres tienen significado especial. Escaparlos evita que el documento deje de ser bien formado.
& becomes &, < becomes <, > becomes >, " becomes " and ' becomes '.
In most places it is allowed unescaped, except in the sequence ]]>. Escaping it everywhere is the safe habit and is what this tool does.
Only the apostrophe. XML defines ' for it, HTML does not. Everything else is the same.
For a long block of text, yes: <![CDATA[ ... ]]> lets you skip escaping. It is not available inside attribute values.
Most control characters (apart from tab, line feed and carriage return) cannot appear in XML 1.0, even escaped. They have to be removed.