Basically, molds consist of 2 main parts: the cavity and core, the cavity side forms the major external surface, core side forms the external features of the molded part, during molding processing; the core and cavity will be separated so the part can be removed. Ideally the part can be molded by the cavity and core directly. But in a lot of occasion, besides the cavity and core, it requires complicated mechanisms when the molded objective is complex, mainly it's because the parts cannot be released completely by 2 directions, it was called undercut, which can't be released by the core and cavity directly, there are lifters and sliders, a series mechanisms are design to release the undercuts, so any success of injection mold depends heavily on the design.
An injection mold is a precision tool and also must be strong enough to withstand thousands of high pressure molding cycles, so the material of the injection mold must be carefully; it's normally special and expensive steel, The way of manufacturing injection mold have been changed along with modern technology developing, main steps are CNC milling, drilling, EDM, wire-cutting, CAD/CAE/CAM technology utilized in a wide range make the lead-time of mold making shorter and shorter.