In structural design of current complex mechanisms or robots like parallel kinematic machines (PKMs), surgical robots, and reconfigurable robots, there commonly exist some functional modules called complex joints (CJs). Each of them, consisting of several simple pairs and essentially a mechanism, plays the same and more important roles as simple joints in kinematics and dynamics. However, as the primarily important aspect in mechanism analysis, the type and mobility of these CJs are far from familiarity. Therefore, this paper aims at addressing the type and mobility of CJs. For this purpose, the concept and classification of CJs are first discussed, an effective method to analyze the mobility characteristics of these CJs is then developed based on the equivalent screw system. The advantage of this method is that it reveals mobility characteristics by using equivalent transformations of kinematic pair screw (KP-screw) and constraint screw (C-screw) systems. With this method, the mobility characteristics of some concrete CJs are obtained correspondingly.