The term template
The term template has a double meaning in TYPO3 CMS. On the one hand, there is the HTML template file, which serves as the skeletal structure in which the content, provided by the CMS, will be rendered. On the other hand, there is the TypoScript template, which is created in the template module in the TYPO3 CMS Backend and can exist on any page.
The Sitepackage Tutorial shows how the two are related together. This manual is purely about TypoScript templates.
Troubleshooting
Common mistakes made with TypoScript templates can cause a message like this:
If you turn on the debug mode you will get more detailed information:
"No TypoScript template found": This warning appears if no template, with the root level flag enabled, is found in the page tree.
"The page is not configured! [type=0][]. This means that there is no TypoScript object of type PAGE with typeNum=0 configured.": This warning appears if the rootlevel flag of a template in the page tree is enabled (i.e. this template is used), but no PAGE Object can be found.
The following TypoScript setup code is enough to remove this warning:
page = PAGE
page.10 = TEXT
page.10.value = Hello World
Do not worry about this code for now, it will be explained later.