Customizable applications play an important role in software as a service (SaaS) scenarios. SaaS application providers want to exploit economies of scale by offering the same basic application to many customers. As customers have different requirements for the same type of application, SaaS vendors must offer so-called application templates that can be customized by their customers to be tailored exactly to their needs. Therefore variability points (i.e., points in an application template that can be customized) need to be made explicit and constraints for these variability points need to be specified. We introduce variability descriptors as a means to describe variability points for SaaS application templates independent of the artifacts (such as GUI components, workflows, configuration files, etc.) that make up the application.