Apache Velocity is a Java-based server-side templating language. ISML templates are contained in your code cartridge and can't be changed without activating a new version of code. But you can change Velocity templates simply by uploading a new version of the file to a WebDAV directory on your instance. Using Velocity templates, you can publish section or page layouts and their content assets to your storefront. You don't have to change your storefront code, effectively decoupling code development and content design.