It's a torque converter. If yours is inverting your torque, you definitely need a new one.
There's loads of symptoms depending on how it's failing. If your engine is making all the right noises, but you're pulling less than you'd expect, then that's the most common sign I think. I think a fluid change can sort that.
As i-CONICA said, there are many symptoms that MIGHT point towards a dodgy torque converter. Details about your car (model, mileage etc) and why you think there is a problem would be helpful.
No disrespect intended but as you don't appear to know what a Torque Converter is how can you suspect its failure?
Lots of help and advice available but it requires some input on both sides