Abstract: Comets are small celestial bodies orbiting around the Sun. They are remnants left over from the formation of the solar system; their interiors store original material of the planetary disk within the solar system. They are thus "fossils" for studying the early solar system. According to their orbital periods, comets are classified as long-period comets (orbital period P > 200 years) and short-period comets (P < 200 years). Long-period comets originate from the Oort Cloud. Compared to short-period comets, they enter the inner solar system less frequently and contain more primitive materials. Studying long-period comets helps us understand the origin of the solar system and reveals characteristics of the Oort Cloud. This paper begins with a summary of our extensive survey of the literature regarding methods of observing comets and foci of comet studies. We introduce systematically the main parameters currently used to assess the activity of long-period comets, including gas production rate, dust production rate, dust properties, morphological characteristics, etc. Subsequently, we discuss in depth the activity mechanisms of long-period comets, covering not only the water ice sublimation-driven mechanism (similar to that of short-period comets) but also various mechanisms that may dominate the activity of long-period comets in the low-temperature environment at the aphelion. These mechanisms include the sublimation of CO or CO₂ gas ice, the polymerization reaction of cyanides, the crystallization of amorphous water ice, the annealing process of amorphous water ice, the thermal decomposition effect, and the electrostatic supercharge phenomenon. We then summarize the evolving activity of long-period comets as they travel from the Oort Cloud to the vicinity of their perihelions. We analyze unique properties of long-period comets, including such special phenomena as changes in dust color, coma structure, and tail structure. Finally, we summarize currently unresolved scientific questions, and then the entire paper.