DOI stands for Digital Object Identifier -- it is kind of like a Dewey decimal system for electronic documents. As far as I know a DOI can be assigned to almost any electronic document but you usually see them on journal articles as a permanent way of referring to the article that won't change even if the web address of the article changes in the future. You can find out more about DOI System at it's offical home page.
So what's so special about the DOI in the title of this post? Well DOI 10.1007/s11168-009-9061-2 uniquely identifies my first journal article. After almost eight years as a researcher I've published lots of conference and workshop papers and one rather weighty thesis, but until Wednesday I'd never had a journal article published.
So here is a direct link to the article which completely breaks the idea of the DOI being separate to the URL at which the document is available. If the link stops work then you should always be able to find the article by resolving the DOI.