Win-win situation is a situation, that is satisfactory to all the parties involved. In win-win situation all the participants can profit in some way, so it is good for each party.
Zero-sum situation is the opposite to win-win situation. In zero-sum situation one party's gain is equivalent to another's party loss. So it is good for one side and bad for other side.
Example of win-win situation in business:
One company is providing webhosting services and the second company is creating websites. These companies are promoting each other. They don't sell the same products, but they have common potential customers.
Example of zero-sum situation in business:
When one company increases its market share, other company decreases its market share.