Sometimes, especially in large textbooks, you find descriptive keywords like "Model Definition" to the left/right of the normal text flow. They help to quickly orient yourself in a lengthy text with many paragraphs. It usually looks like this:
and therefore we now define the model.
Model Definition A good model covers all the aspects of
the real problem, but ignores the de-
tails. [...]
What are these "keywords" called? An what package allows me to define them for my own text?
\marginpar{<text>}. Of course some packages will give you further options and more semantic commands (for example for keywords, headers, remarks...). – T. Verron Mar 08 '13 at 07:45:)– T. Verron Mar 08 '13 at 10:27