String Theory is expected to provide the framework of quantum spacetime combining the principles of relativity and quantum theory. Through the revolutionary developments in the string theory since 1994, many deep and new properties of the theory have been discovered. The most important newly discovered property is the so-called AdS-CFT correspondence or the holography principle. This relates the strongly interacting theory with the corresponding effective gravity theory in one dimension higher spacetime. In other words, the matter interaction can be described in terms of gravitational theory of the general relativity. The duality concept will be introduced while reviewing particles and solitons. After brief reviews on the strings, the holographic idea as an ingredient of the new paradigm will be explained with some examples.