A very simple explanation of how Zotero does what it does:
Zotero “senses” information on the page by reading the coding language that has been used to identify particular pieces of information. The language labels pieces of information and Zotero identifies these labels, copies the labelled information into your library, and then uses the labels to sort the pieces of information into the various citation formats. Currently, there are two coding languages used for making information accessible in such a manner: XML and RDF. If you’re still curious, you can learn more about each of these languages at:
