Scratch is a programming environment that allows you to describe the behaviour and looks of objects on the stage.  The stage is likened to a window  on a world where these objects reside and interact with you, the environment and other objects on the stage.

Objects, their looks and behaviour, is driven by the Scenario - or situation in which those objects exist.  An object's looks and behaviour will be different depending on the circumstances in where we find those objects.

For example, a Cat in a comfortable home where it is pampered and looked after would look and behave differently from a Cat that lives  on the streets where it has to survive scrounging for food and looking after itself.

As a Scratch Programmer, your job is to describe the looks and the behaviour that would be appropriate for the objects you are building depending on the Scenario you are presented with.

Creative Computing - Scratch Programming Lessons