Many journals now include DOI numbers as part of their articles’ citations. A DOI (Digital Object Identifier) is a unique number that will help you find an article at any point in the future, even if the address of its webpage has changed.
If you see a DOI in a citation you can go to http://dx.doi.org/ and type the DOI into the box on this webpage. You will then be redirected to the current online location of the article you are looking for.
To find the example in this image you would go to http://dx.doi.org/ and type 10.1038/nature09068. This would take you to the current online location of the referenced article.