is one that is easy for a program to read/understand - properly put together in other words.
From your question I suspect that you're asking about XML rather than HTML. XML is used for the 'back end' aspects of a web application, and HTML for the front end. We're wading out into deeper water here, and this isn't a subject that can be adequately tackled in one answer.
To be 'well formed' an XML document must meet the following criteria
1. The XML declaration must come first in every document.
2. Comments are not valid within a tag.
3. Comments may not contain two hyphens in a row, other than the beginning and end of the comment.
4. Tags must have an end tag, or be closed within the singleton tag itself, for example:- <br />
5. All attributes of tags must be quoted, preferably using double quotes unless the attribute itself contains a double quote
6. Every XML document must contain one element that completely contains all the other elements
To see some further reading on this subject