Direct-ascent utilized a single launch vehicle and one craft to land on the Moon and later return to Earth. This mode required no docking maneuvers in space, but did require a larger rocket than the one then in development-the Saturn V. Such a rocket would have been extremely difficult to complete by 1970.
EARTH ORBIT RENDEZVOUS (EOR)
EOR required launching the lunar spacecraft in pieces aboard several rockets and assembling them in Earth orbit. Each piece served a particular function during the mission and would then be discarded.
LUNAR ORBIT RENDEZVOUS (LOR)
LOR required several craft, sent up on a single launch vehicle. Over the course of a lunar journey, each craft performed a specific part of the mission. After attaining lunar orbit, the lander separated from the main craft and descended to the surface. After performing its function on the surface, part of the lander lifted-off for rendezvous with the orbiting ship which returned to Earth, leaving the lander in lunar orbit.