Mars is our Earth's neighbor with many aspects very much similar to the Earth, Mars is sometimes considered as the future Earth, which means that the Earth's internal structure, surface, and space environment, may evolve in a long period, to those similar to the present Mars. Therefore, based on a comparison with the Earth, to explore and investigate Mars is important not only to the revelation of the universe mystery, but also to better understanding of our Earth. Since the 1960's, the spacecrafts have been sent to explore Mars. There were more than 40 spacecrafts (with some failures) to explore Mars and some important results were obtained in the last more than 40 years. These results help us understand Mars, such as the Martian internal structure and surface, atmosphere, ionosphere and magnetosphere, and so on. This paper analyzes the results of the exploration and investigation on Mars, including the Martian internal structure, surface, atmosphere, ionosphere, intrinsic magnetic field and magnetosphere, as well as the solar wind interaction with Mars. A brief introduction on the main scientific questions and significances of the Mars exploration and investigation is also presented.