Plasma's vision statement, as developed and agreed by its core contributors, is:
"Plasma is a cross-device work environment by the KDE Community where trust is put on the user's capacity to best define her own workflow and preferences.
Plasma is simple by default, a clean work area for real-world usage which intends to stay out of your way. Plasma is powerful when needed, enabling the user to create the workflow that makes her more effective to complete her tasks.
Plasma never dictates the user's needs, it only strives to solve them. Plasma never defines what the user is allowed to do, it only ensures that she can.
Our motivation is to enable actual work to happen, across devices, across different platforms, using any application needed.
We build to be durable, we create to be usable, we design to be elegant."
sebas' blog has more info on its background and explains its parts.