Difference between a volume and a cluster

Dear list,

This question has been bugging me for quiet a while.

What are the exact definitions of "clusters" and "volumes", and what- if any- are the difference between the 2?

Google search is not yielding any results:

Difference between a cluster and a volume : 0 results Difference between a volume and a cluster : 0 results
Difference between clusters and volumes : 0 results
Difference between volumes and clusters : 0 results

The search is continuing however.

Eagerly awaiting responses.

Thanx in advance

SNODX