Odoo views are designed and maintained in XML, which is short for Extensible Markup Language. A basic understanding of XML will help you in customizing views, modifying search criteria, and managing workflows. Here is a list of XML resources.
This is the official page for the XML specification, http://www.w3.org/XML/.
If you are just beginning to learn XML, make sure that you visit this excellent collection of tutorials and documentation at http://www.w3schools.com/xml/.
RML or Report Markup Language is a specialized XML format utilized for reporting. Odoo uses RML for most forms and simple reports inside of Odoo. Understanding RML basics will help you in designing reports or modifying headers and footers of reports, which are available at http://www.reportlab.com/docs/rml-for-idiots.pdf.