The DOM Level 1 Recommendation is at http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/. Unlike most standards, this one is actually pretty readable, particularly if you're only interested in the Core XML interfaces.
Level 2 of the DOM has also been defined, adding more specialized features such as support for XML namespaces, events, and ranges. DOM Level 3 is still being worked on, and will add yet more features. The World Wide Web Consortium's DOM page at http://www.w3.org/DOM/DOMTRhas pointers to the specifications and current drafts for these higher DOM levels.