1. A network-based robot system comprising: an activity creating device creating activities in hierarchical structure, each activity describing action execution procedures to be carried out and being arranged in an activity tree; a robot downloading the activity tree over a communication network and executing an activity selected in the activity tree, wherein the robot comprises: an activity storage storing the activity trees provided through the communication network; an activity manager searching for, in response to an activity execution instruction, an activity tree having an activity corresponding to the execution instruction from the activity storage; an activity interpreter interpreting the activity tree searched by the activity manager; an activity executor selectively executing activities in the interpreted activity tree; and a plurality of primitive actions to be taken by the robot.