Forum OpenACS Q&A: Response to Newbie Alert: ADP vs TCL

Or you could see the .adp files as the templates where you store your design and layout and the .tcl files where you store your logic. In that way you achieve the holly grail of CMS of separation of content/logic/layout.

I prefer to see it that way so that as soon as you start coding you have that idea in mind and it's really important (at least I like to think so ;)