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 | 110This gate may or may not be related to quantum computing, but is often mentioned in that context.