index.php is the main script for showing pages with TYPO3 / TypoScript. This page show some information about this script and how to use it.

Normally you request pages by setting a value for "id" and possibly "type".

"id" refers to a page. This is an integer. If a string is supplied, it's regarded as an alias and the corresponding page is found.

"type" defines which "type" the page is. Always an integer (0-255). If "type" is not set it's regarded to be zero. "type" is used to build framesets. Fx. the frameset would have "type=0" (or nothing) and the pages in the various frames would have "type=1" and "type=2" and "type=3". In TypoScript you define a PAGE-object for each type so TYPO3 renders different pages depending on the type-value. Normally the PAGE-object displaying the page content is named "page" and has  the "type=1" value.