dynamic template layouts?

[Don Smith]

I have a question about how to substitute different content in a site
template. For example, I have a template that has a header, footer,
body and sidebar. This template is referenced in my outer most site
tag. However, depending on where a user navigates I want to change
what is in the sidebar. Is there a hook to be able to make that
dynamic, or do I need to combine it with the body and define the
sidebar in each page template?

Thanks,

Don