For which Websites is the law valid?
The imprint is described in the Telemediengesetz (TMG), § 5 Allgemeine Informationspflichten (german text). It is viable for "businesslike pages". Though it isn't defined exactly what "businesslike" means and the term is open to interpretation, it is recommended to consider this regulation on all pages.
Where should the imprint be placed?
The imprint has to be recognized easily and directly by every visitor. It is recommended to use the terms "imprint" or "contact".
The imprint has to be found very distinguishable. The user should not make an extra click, not scroll horizontally or down to a fourth subpage. Links that are close to the imprint should not be named mistakable (e.g. "about").
Furthermore, it has to be added as a text file (not as an image) and has to be in the language of the website.