From ProtoScript

ProtoScript is a scripting language for creating prototypes for the Web. It allows user interactions to be expressed in a simple readable style. By clearly exposing Interface Elements (e.g., a drop target, an item in a list or a complete set of objects), Interaction Behaviors (e.g., Fade, Spotlight, Hide) and Event Callbacks (e.g., when a fade completes, when the user clicks on something), behaviors can be tied to user interface elements when certain events or situations occur in the interface.

Additionally, ProtoScript is built to be independent of any specific Ajax framework. By using a plug-in architecture, the way interface elements are selected is controlled by which selector technology gets plugged into ProtoScript (e.g., jQuery, CSSQuery, MooTools). A specific Behavior set (e.g. YUI, Dojo, MooTools) can be built and plugged into ProtoScript allowing the generic language construct to work with any toolkit.