see here for a full list of named entities

How to read it:

<!ENTITY iexcl CDATA "&#161;" -- inverted exclamation mark, U+00A1 ISOnum -->

<!ENTITY starts an entity definition
iexcl is the name of the entity so you can use this entity by writing &iexcl;
CDATA is the data type of the entity - of no interest...
"&#161;" shows what the named entity is transformed to - you could always use &#161; instead of &iexcl;
-- starts a comment
inverted exclamation mark, U+00A1 ISOnum describes the entity
-- ends a comment
> ends the entity definition

Not all browsers do understand all named entities.

And even if they do understand it, it depends on the current font,whether the character is available or not.

(Now lets hope I did not mess up the HTML in this post...)