EntryClass
Pages and entries are technically very similar to one another.
In fact most,
if not all,
of the <mt:Entry*
> tags will work for publishing pages and vice versa.
Therefore,
to more clearly differentiate within templates between pages and entries the <mt:EntryClass> tag returns one of two values: "page" or "entry" depending upon the current context you are in.
Example:
<mt:If tag="EntryClass" eq="page"> (we're publishing a page) <mt:Else> (we're publishing something else -- probably an entry) </mt:If>