What is the difference between an exchange pool and a reservoir? what role do these areas play in the water cycle?

EXCHANGE POOL is a part of the cycle that holds water for a short period of time. 

RESERVOIR POOL is a part of the cycle that holds water for a long period of time. 

So, the main difference is the period of time each one holds water during the water cycle. 

Explanation:

Water is recycled during the water cycle. Chemical elements and water that are important for an ecosystem and the animals living in it are CONSTANTLY being recycled.  For instance, chemicals might travel from organisms to the atmosphere or oceans and come back to the organisms again. But, it certainly happens that water is held for many years in different parts of the cycle. For example, the atmosphere plays the role of an exchange pool because it holds the water in the shape of vapor for just a few days. The ocean is a reservoir for water since it may hold water for thousands of years. The water cycle happens on, above and below earth surface, so the water cycle is a global cycle.