Macros are delimited with percent signs inside angle brackets.
You can define your own macros.
Example: <%includeHTTP ("http://abc.com/")%>.
Here's an example page that uses several macros.
<%slogan%> -- returns one of 15 slogans.
<%now%> -- to say what time it is on the server.
<%text%> -- uses the text of the parent node to give the page a title.
<%pickleFlavor%> -- a custom attribute from the parent node.
A blog post with some more examples of macros.
You can also define your own macros.