3D printing, or additive manufacturing, is the process of depositing layers of material to build up the desired geometry. Generally speaking, CNC machining refers to subtractive processes where the material is removed from the workpiece, resulting in the desired geometry. While 3D printing processes can create more complex geometry like internal latticing, parts need support during printing to prevent failure. CNC machining generally produces more accurate parts and can be significantly faster, especially when manufacturing single parts.
Many manufacturers are using additive and subtractive manufacturing together. In simple cases, they 3D print a part, then use CNC milling to machine critical features such as holes, slots, and mating faces that need additional precision. In more niche applications, hybrid manufacturing can inter-weave additive and subtractive processes to combine the best of both technologies. For example, a manufacturer may print a hollow part and include CNC machining steps at regular intervals throughout the 3D print process instead of doing all the printing followed by all the CNC machining.