Templates are built on classes, however sometimes you could switch to a similar base class. Questions about templates don't have to be class specific.
Very different templates for different purposes can use the same class.
Templates can provide additional features in different ways. I expect that questions about templates are about such additional features, not about class features.
In contrast to classes, some templates come with a prebuilt text structure, such as certain sections or chapters in a prepared order for users to fill in text. Thesis and some letter templates may be such examples.
Templates may consist of several files or even a prepared directory structure, such as a main document, a preamble file for \input, chapter files for \include, image directory, etc.