XML data is used for data interchange in the financial and insurance industry so if you work in these industries, there is a good chance you will come across some format at this point. No predefined schemas are used in XML, but that also means that there is a fair amount of data parsing that needs to be performed after the data is read in order to extract the data itself. The XML() package is used to read XML data into R.
JSON is another standard derived from web technologies that was originally used to transmit sets of key pairs between different application systems. The jsonlite package can be used to parse JSON files.