KDE 4.5 has been released some weeks ago. We take this occasion to interview Aaron Seigo (aseigo).
Ikhaya: Could you please explain, to our readers from the Ubuntu world, in a few sentences what Plasma is?
Aaron: Plasma is a KDE technology for creating desktop environments. It combines a world class window manager which has advanced features such as focus stealing prevention, window tabbing and hardware accelerated desktop effects with a workspace that is made up entirely of "widgets" (analogous to apps on a smart phone, really) along with all the essential tools needed to get the most out of the computer, such as a system settings center.
Using these pieces, we create and ship products such as the Plasma Desktop which provides a modern, usable desktop environment for larger computers, or Plasma Netbook which is designed for devices with smaller screens, driven by touch (it works great on tablets, for instance) and used primarily for information access. These different products share almost all of the same code, allowing components written for one system to run well on all Plasma systems. This can be as simple as a battery indicator or as complex as a Twitter / identi.ca client.
Part 1 
Part 2