Workflow is a business logic. Workflow in salesforce is basically a set of rules and criteria that will automate an action like sending an email, creating or updating a record etc..
If the defined criteria is right then action will be executed, or else not. In workflow there are two types of actions can happen.
1)Time dependent actions
2) Immediate Actions.
The different actions are:
1)A field update,
2)sending an email,
3)sending outbound message,
4)Assigning a task to someone in salesforce organization.
In a nutshell, workflow is a business logic.