HTML is a markup language which allows you to define and place elements on a page, or Document Object Model (DOM). Of course, HTML is static. That means that when a web designer uses a certain HTML attribute to make a word bold, for example, there is no way with HTML alone to change that setting. One part of JavaScript allows a programmer to add/remove or modify the DOM elements on the fly using scripts and stylesheets known as CSS. The combination of HTML, JavaScript and CSS is collectively known as DHTML (Dynamic HTML).
Note that Javascript is a fully developed object based scripting language and can do many other things aside from DOM manipulation.