Nuclear winter is the idea that the pollution from the resulting fires will produce aerosols which reflect sunlight and cause cooling, with a dramatic impact on life on Earth (apart from the radioactivity). Something similar has occurred in the past with the large bolide (comet) that collided with the Earth at the end of the Cretaceous (KT event). Large scale collapse of photosynthesis saw the demise of many large animals, including the dinosaurs.
A recent comment in Nature shows that even a small scale conflict could have dramatic effects in the form of a nuclear winter - say for example India and Pakistan. The author notes that advances in computer models have shown that the problem is worse than first understood:
"By 2007, models had began to approximate a realistic atmosphere up to 80 kilometres above Earth's surface, including the stratosphere and mesosphere. This enabled me, and my coauthors, to calculate for the first time that smoke particles would be heated by the Sun and lifted into the upper stratosphere, where they would stay for many years. So the cooling would last for much longer than we originally thought."
To go back to the India and Pakistan case, if 50 Hiroshima-size bombs were dropped on the largest cities in each country, five million tonnes of black carbon smoke would be emitted into the upper troposphere and then into the stratosphere by solar heating. Temperatures would be lower than during the 'Little Ice Age' (1400–1850), during which famine killed millions.
So in short, no nukes is good nukes!