The output is high when either of inputs A or B is high, or if neither is high. In other words, it is normally high, going low only if both A and B are high.
The NAND gate and the NOR gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate or an AND gate. The non-inverting gates do not have this versatility since they can't produce an invert.
The NAND gate is called a universal gate because combinations of it can be used to accomplish all the basic functions.
Suppose you want a high output when either A or B is high but C is low. The boolean expression and straightforward gate version of this are:
But the same task can be accomplished with NAND gates only since NAND's are universal gates. Integrated circuits such as the 7400 make this practical.
Index
Electronics concepts
Digital Circuits