Laser communication links in space are attractive alternatives to present-day microwave links. This tutorial describes the basic concept and the functions of an optical terminal on board a spacecraft. It points out the differences between free-space optical links on one hand and glass fiber systems and microwave directional links on the other hand. The requirements on data transmitters and receivers as well as on optical antennas and pointing, acquisition and tracking mechanisms are discussed. Typical application scenarios are outlined, experimental systems and their technologies are cited.