Logic Gates are the building blocks for digital circuits, and they are comprises transistors. The transistors act as on-off switches. 

Logic Gates perform logic or Boolean functions defined by truth tables, they have only two values, 1 or 0.

The 3 basic logic operators are NOT, AND, OR

The 3 basic logic gates are NAND, NOR, INVERTER

A CMOS logic circuit is in effect an extension, or a generalization, of the CMOS inverter: The inverter consists of an NMOS pull-down transistor, and a PMOS pull-up transistor, operated by the input voltage in a complementary fashion.

The CMOS logic gate consists of two networks: the pull-down network (PDN) contructed of NMOS transistors, and the pull-up network (PUN) constructed of PMOS transistors. The two networks are operated by the input variables, in a complementary fashion.

Thus, for the three-input gate, the PDN will conduct for all input combinations that require a low output (Y=0) and will then pull the output node down to ground, causing a zero voltage to appear at the output. Simultaneously, the PUN will be off, and no direct dc path will exist between Vdd and ground. On the other hand, all input combinations that call for a high output (Y=1) will cause the PUN to conduct, and the PUN will then pull the output node up to Vdd. Simultaneously, the PDN will be cut off, and no dc current path between Vdd and ground will exist in the circuit.