What is difference between Visual Workflow and Workflow Rule in Salesforce?

Hello Shubham,
You can call the flow when the user clicks on a custom button/link ,A Process starts ,Apex is called ,Inline Visualforce page ,The user accesses a custom tab. Flows can execute logic, interact with the Salesforce database, call Apex classes, and guide users through screens for collecting and updating data. You can take a look here Figure Out Which Tool To Use

I will suggest following the awesome blog series to getting started with flow here

Learning Flow

Workflow
Workflow enables you to set up workflow rules. A workflow rule identifies what kinds of record changes or additions trigger specified workflow actions, such as sending email alerts and updating record fields.
Workflow rules and actions are associated with a specific object (and can cross objects only to update fields on a related master record).

Visual Workflow
Visual Workflow enables you to create flows, which are triggered by users rather than events. Unlike Workflow, which always executes rules and actions behind the scenes, Visual Workflow offers screens for displaying and collecting information from the user running the flow.

Thanks