What does it mean to describe the foreignexchange market as an “over-the-counter market”?

1. What does it mean to describe the foreignexchange market as an “over-the-counter market”?
2. What is the difference between a spot transaction and a forward transaction in the foreignexchange market?
3. What are the key differences between foreignexchange forward contracts and foreignexchange futures contracts? Why are forward contracts more widely used in the foreignexchange market than are futures contracts?