ZK Class (aka., zclass) is a naming pattern. The name assigned to zclass (HtmlBaseComponent.setZclass(String)) will be used to name the CSS classes associated with the DOM structure of a component, including the root and the children. In addition, each kind of components is assigned with a unique zclass and shipped with all the required CSS rules.
Since zclass is used to name the CSS classes associated DOM elements, all the default CSS rules won't be applied if zclass is assigned with a different value. Thus, it is used to custom a component with a totally different look.
For example, assign zclass btn to a button component in zul page like this
<!-- index.zul --> <button zclass="btn"/>
will generate the following output
<!-- HTML output --> <button class="btn" />
<!-- ZK default HTML output --> <button class="z-button" />
As you can see the default z-button class is missing, which means all CSS rules is removed.