The Online Library is your virtual bookshelf. It contains all the publications listed, in unabridged form, just like a printed book or the pdf version.
The Mediathek stores extra resources that are relevant to your DIN Media publications and can be directly downloaded. These may be additional texts, pdfs and/or flash, video and audio materials, depending on which publications you have bought from DIN Media. You have to register separately for the Mediathek.