The Digital Object Identifier system is a way to identify content objects in digital networks. Among its advantages are that the DOI number is relatively short (compared to many URI on websites) and that they are stable (so much less likely to suffer from link rot).
A lot of academic resources (journal articles specifically) are most reliably linked to using their DOI number.
Minor feature request for the academic SE sites (like physics or maths): Can we support a link syntax for DOI numbers? I'm imagining something like
[DOI:1234.5678/901] getting turned into what would be displayed by