There are two Workflow types available in Salesforce:
1. Immediate Workflow rule: Workflow rule fires immediately when the workflow criteria is met, and the associated actions (email alert/field update etc.,) will take place immediately.
2. Time Dependent Workflow rule: When the workflow entry criteria is met, associated actions (email alert/field update etc.,) will take place after a certain period of time. This time is based on the value that you set.