Understanding of these two concepts is crucial since any business process ultimately can be decomposed into a hierarchical structure of mixed operations and rules.
A workflow is to describe a business process, an orchestration (ordering or sequencing) of a collection of tasks or operations. A workflow also ensures those tasks are carried out by the right actors, in the right time, and in the right sequence etc.
Business rules, on the other side, are criteria imposed on business operations to guide the behaviour, to shape judgements, and to make decisions. Structurally, a business rule consists of a set of conditions as well as business actions/activities (smaller behavioural units than operations or tasks), and when the conditions are met the actions are to be taken. Rule-sets, as the name tells, is a tool to group related rules.
An example of business rule:
If a car has no airbags then car 'rating' of 'potential occupant injury' is 'extremely high'