Today, you took a look at how to create well-formed XML documents. W3C doesn't even consider an XML document to be XML unless it's well-formed. W3C considers an XML document well-formed if it meets three criteria:
Taken as a whole, it matches the production labeled document.
It meets all the well-formedness constraints given in this specification (that is, the XML 1.0 specification, http://www.w3.org/TR/REC-xml).
Each of the parsed entities, which is referenced directly or indirectly within the document, is well-formed.
The most general of these items says that an XML document must meet the well-formedness constraints in the XML specification, and you took a look today at what that meant.
Those constraints include beginning a document with an XML declaration, using only legal character references, the document must include at least one element, elements must be structured and nested correctly, the root element must contain all other elements, attribute names must be unique, attribute values must be quoted, and so on.
You also took a look at creating namespaces, and how namespaces help you avoid conflicts in XML. To define a namespace, you can assign the xmlns:prefix attribute to a unique identifier (usually a URI), or you can use the xmlns attribute to define a default namespace.