The file is includes/languages/english.php

Make sure you copy it into includes/languages/YOUR_TEMPLATE/english.php before making any changes. You'll also need to set your admin settings to your custom template if you haven't already done this.

You can edit the title, tagline & custom keywords (as well as many other things in this file) by editing any text inside the 'quote marks'. If you want to remove anything, leave it blank ' ' rather than deleting it. Make sure also to remove the slashes // in front of the lines you edit (if there are any) because they comment out any line they're in front of.

Hope this helps, and that I didn't tell you too much that you already know...