What are the differences between UMLl and HTML?

UML (Unified Modeling Language) is a diagramming "language" used primarily for software design. It is called Unified because it standardizes several slightly disparate elements (e.g. uses cases, class diagrams, sequence diagrams).

XML (eXtensible markup language) is a language for storing and exchanging data in software, such are storing documentation or passing messages between applications. The Word "extensible" is used because the language itself has provisions for creating and using user defined "tags".