A Document Type Definition (DTD) is a document that allows a program, such as a web browser, to properly parse a markup language that is formatted according to a specific use. HTML and its XML-based brother XHTML are probably the most common DTDs today, supplying web browsers with the guidelines for properly rendering web page elements.

Further Reading

W3C Definition of DTD

A markup language is a set of tags that describe the content of a document. The most common markup languages today is HTML a markup language is not a programming language, hence it is a misnomer to 'code' HTML. XML has introduced extensibility into HTML called XHTML. This new more versatile language can incorporate many types of media, other than plain text; and web browsers are constantly adding more support for W3C languages such as SVG and XSLT. XML has strict rules that can increase the learning curve for a person new to XML-based markup.

Here are some development tips and tricks I've found useful over the years.