The second part of the International Space Station, The American Unity, was launched on December 04, 1998 with the Space Shuttle Endeavour. The main job of Unity is to keep the comming parts of the Space Staion together. (Unity has six docking ports). To both ends of the zilver colored Unity are 'Primary Mating Adapters' (PMA-1, top on the photo and PMA-2, bottom of the photo). After astronaut Nancy Currie had lifted Unity with the robot-arm from Endeavour's cargobay, she docked it with PMA-2 to the docking-ring of the Shuttle.
After that the Shuttle rendezvoused with Zarya and with the arm of Endeavour astronaut Currie placed Zarya on PMA-1. PMA-2 will, until the next American Module "Destiny" is put in orbit, be used as docking port for the Space Shuttle.
On the white docking port of Unity (on the photo) comes in 2000 ?? the Z1-trust, the base for the solar pannals of the Station. On the docking port on the other side of Z1 comes PMA-3, wich will be used as docking port for the Shuttle, when PMA-2 is being replaced. The other two docking ports of Unity will later be used as Airlock for EVA's and a glass observation post ('Cupola').