From the longer Wikipedia page 
Diplomatics (in American English, and in most anglophone countries), or Diplomatic (in British English), is a scholarly discipline centred on the textual analysis of documents – particularly, but not exclusively, historical documents. It focuses on the conventions, protocols and formulae that have been used by document creators, and uses these to increase understanding of the processes of document creation, of information transmission, and of the relationships between the facts which the documents purport to record and reality.
The discipline originally evolved as a tool for studying and determining the authenticity of the official charters and diplomas issued by royal and papal chanceries. It was subsequently appreciated that many of the same underlying principles could be applied to other types of official document and legal instrument, to non-official documents such as private letters, and, most recently, to the metadata of electronic records.
Diplomatics is one of the auxiliary sciences of history. It should not be confused (as it often is) with its sister-discipline of palaeography. In fact, its techniques have more in common with those of the literary disciplines of textual criticism and historical criticism.
In the ancient and medieval periods, the authenticity of a document was considered to derive from the document's place of preservation and storage, in, for example, temples, public offices, and archives. As a result, those with nefarious motives were able to give forged documents a spurious authenticity by depositing them in places of authority. Diplomatics grew from a need to establish new standards of authenticity through the critical analysis of the textual and physical forms of documents.
Diplomatics became important during the Reformation and Counter-Reformation. Its emergence as a recognisably distinct sub-discipline, however, is generally dated to the publication of Mabillon's De re diplomatica in 1681. Mabillon had begun studying old documents with a view towards establishing their authenticity as a result of the doubts raised by the Jesuit Daniel van Papenbroek over supposed Merovingian documents from the Abbey of Saint-Denis. During the Middle Ages, the production of spurious charters and other documents had been common, either to provide written documentation of existing rights or to bolster the plausibility of claimed rights. Mabillon's work engendered a far livelier awareness of the potential presence of forged or spurious documents, in the fields of both history and law.
Although Mabillon is still widely seen as the "father" of diplomatics, a more important milestone in the formation of the battery of practical techniques which make up the modern discipline was the publication of René-Prosper Tassin and Charles-François Toustain's Nouveau traité de diplomatique, which appeared in 6 volumes in 1750–65.
The most significant work in English was Thomas Madox's Formulare Anglicanum, published in 1702. In general, however, the discipline was always studied more intensively by continental scholars than by those in Britain.
Diplomatics is often associated with the study of documents of the medieval period. However, scholars such as Luciana Duranti have argued that many of its theories and principles can be adapted and applied to contemporary archival science.
More information here .