A toffoli gate is supposed to be a universal reversible logic gate, which probably means that any reversible circuit can be constructed from toffoli gates.

The gate has a 3 bit input and output. If the first two bits are set, it flips the third bit. Following is a table over the input and output bits:

 in | out
000 | 000
001 | 001
010 | 010
011 | 011
100 | 100
101 | 101
110 | 111
111 | 110
This gate may or may not be related to quantum computing, but is often mentioned in that context.