There's currently no documentation other than a basic overview of use of Wordlets that should be provided on the IP website.
I guess we'll have to step in with descriptions.
Wordlet is an Infopop-ism. They're a type of what is normally called "language files". Individual strings are stored seperately, changable by the cpanel. Much of the included UBB text is done via wordlets. On this reply form, for instance... "Message Icon", "Message", "HTML is not enabled", "UBB Code is enabled", etc.
Templates are a little more difficult to explain.
Perl has the ability to read and execute a script at the command of another. This is done using a few different methods. The most often used is the require() function. UBB's templates are really nothing more than Perl subroutines that return HTML code. That HTML code is quite easy to find and edit, as there's almost no Perl code to get in the way.