Technology tree
From Kerbal Space Program Wiki
The Tech tree is the screen used in the R&D center to unlock bigger and better parts as the player does research in space. (As this is an initial release of this feature, new with version 0.22, this is subject to change.) The Tech tree consists of multiple layers of research: the branch of rocket design you are currently exploring such as Stability or Survivability, and development: the parts unlocked with each new completed research.
When you start you "own" knowledge of only a few parts. As you complete research, more parts become available. The Tech tree lists the reaserch available at each step, and also the parts owned when completed, is as follows (table under development):
