The earth's early history and geological processes at depths are the two frontiers of geosciences both of which related to granitic rocks (granitoid). Granitic rocks mainly consist of quartz, plagioclase and alkaline feldspar and hence have lower densities. This feature decides the granitic rocks have positive buoyancy and the long history records of the earth's evolution. Therefore, granitoid is one of the most difficult topics in geology due to its complex sources and formation processes. This paper is used to help general tourists to recognize granitic rocks and their importance. Generally, the term granite is defined as a pluton which consists of quarts (>20vol %) and feldspar (alkaline feldspar > plagioclase). However, geologists often call the rocks with features similar to the granite as granitic rocks or granitoid. Therefore, the granitoid is a big group of plutonic rocks including such as granodiorite, monzonite, syenogranite, tonalite, and so on. The igneous diversity is not only induced by the magmatic processes but also by the processes caused production of magmas. The generation of magma is resulted from partial melting of the source rocks and the igneous chemistry is the function of the source composition, melting temperature and pressure, volatile, and melting degree. Despite the important effects of magmatic processes on the igneous compositions, the partial melting is the key of the diversity for the granitic rocks. Based on this, the petrologists pay more attentions to the origin of magmas instead to the evolution of granitic magmas. The granitic magma generated in the crust, but the generation is driven by the mantle thermal engine. So, granitic rocks recorded a great of information about the geological processes in the depths of the earth.