What is a contact role and for which objects is it available?

A contact role defines the part that a contact or person account plays in a specific account, case, contract, or opportunity. For example, Tom Jones might be the Decision Maker for the opportunity, and Mary Smith might be the Evaluator. You can assign a contact role to any contact or person account that affects your account, case, contract, or opportunity. Contacts and person accounts can have different contact roles on various accounts, cases, contracts, or opportunities.