Suppose you have some google adsense code which you want to have display in categories, but not on other pages. Yet you want to have this code in your wrapper template, not in the 'show links in category' template. To do this, you can use a template conditional (see the conditionals manual entry if you don't understand what a conditional is), like this:

<IF templates/default/details.tpl contains displaylinks.tpl>
stuff that should show up when we're looking at a page that uses displaylinks.tpl (the 'show links in category' template)
</IF>

Or for another example, to have something show in the wrapper only on the registration page:

<IF templates/default/details.tpl contains register.tpl>
stuff that should show up when we're looking at a page that uses register.tpl (the 'registration' template)
</IF>

templates/default/details.tpl, as you can guess from the examples, contains the name of the body template the page is using.