Mechanisms
Precipitation occurs when air is saturated; this occurs when air rises; which in turn usually occurs in one of three ways.
Convective precipitation occurs when air rises vertically through the (temporarily) self-sustaining mechanism of convection. Stratiform precipitation occurs when large masses over air rise slant-wise as larger-scale atmospheric dynamics force them to move over each other. Orographic precipitation is similar, except the upwards motion is forced when a moving airmass encounters a rising slope.
Read more about this topic: Precipitation Types