According to channel classification: field effect transistor is divided into PMOS transistor (P channel type) and NMOS (N channel type) transistor. 

Classification by material: can be divided into depletion type and enhancement type: 

Enhancement transistor: drain current is zero when gate-source voltage Vgs is zero; Depletion mode transistor: drain current is not zero when gate-source voltage Vgs is zero. 

In fact, to sum up, there are four types of MOS tubes: 

Enhancement PMOS, enhancement NMOS, depletion PMOS, depletion NMOS. 

In practical applications, enhanced NMOS and enhanced PMOS are the main ones. So NMOS and PMOS are usually referred to as these two kinds. Combining the following figure with the above content can also explain why the actual application is mainly enhanced, mainly when the voltage is 0, whether the D pole and S pole can be conducted.