Logic Gates

["Bart"]

I look at all the symbols for the different logic gates, NAND, NOR, XNOR,
etc. and on some of them they have a little circle on the output. I look at
datasheets for different IC's and I see the same little circle on some of
the pin designations. My electronics books show these but I can't find what
it means.
Any help is appreciated,
Bart