An XML document is WellFormed if it parses correctly. The main issue that causes XML not to be WellFormed is tag ordering. For example the following is WellFormed

... <a> this is well formed </b> <c> this is well formed too </c> </a> ...

while this is not

... <a> this is well formed <b> <c> this is well formed too </a> ...

In the second fragment, neither the <b> nor the <c> tag are closed. Unlike SGML, XML does not allow tags to be automatically closed when the enclosing tag is closed. This is the reason why the <p> tag in HTML/XHTML gives people grief---in HTML you only need to put in the opened tags while in XHTML you need to put in both the opening and the closing tag.

The tag pattern

<a> ... ... </a> ...

is not WellFormed in either SGML or XML, if you want non-nesting overlapping ranges such as this you need something like

<a id="1"/> ... <b id="2"/> ... <a id="1"/> ... <b id="2"/>

then you can reconstruct either of the tags as necessary.

WellFormed XML differs from Valid XML in that Valid XML has been (or could be) checked against a Schema or DTD.