Comprehension of complexity is one of the key challenges in science and technology. Complexity is a very important conception in interdisciplinary systems science, and is significant to researchers. The basic semantic background on system complexity was presented and the various definitions of complexity domestically and overseas were analyzed. On the basis of the classification of ontology and epistemology, a taxonomy of complexity was described on the basis of the state-of-the-art works on complexity definition. Meanwhile, a category of complexity measurements and the corresponding mathematical tools were proposed. Examples were given for the efficiency of complexity classification and measures.