Using multiple index pages

I would like to use two PHP templates: one for the main index.php, and one for the individual post pages. The purpose is to have different text displayed in the sidebar for the post pages than for the main page of the site (a ‘what’s this?’ text).
There are a few threads here that explain how to use different CSS files for different pages, but this is not what I need. The design will be the same between the main index.php and the individual post pages – it’s the content of the pages that I’d like to be different.
Do any of you have any ideas?
Thanks,
M.