A NAND gate is a logic gate that produces a false when all of its inputs are true. For all other combinations, the output will be true. Table 1 shows that truth table for a NAND gate. The output of a NAND gate is the complement of that of an AND gate. NAND is made from Not and AND. Therefore, you can build a NAND gate by connecting a NOT gate and a NAND gate together, as shown in figure 1.
The NAND gate (along with the NOR gate) is very useful because it can be used to create any other boolean function! This is known as functional completeness.
NAND gates are often seen with 2 inputs but they can have any other number of inputs that are required.
TTL and CMOS versions of NAND gates are popular choices, see table 2.
There are 2 different symbols for a NAND gate, as shown in figure 2. The ANSI symbol is the most commonly used one.