What are the differences between Workflows and Process Builder in Salesforce?

Hi Suryadeep

You can use the Process Builder to perform these actions :
Update any related record
Use a quick action to create a record, update a record, or log a call
Launch a flow
Send an email
Post to Chatter
Submit for approval
Call apex methods

Workflow does only 4 actions:

Create Task
Update Field
Email Alert
Outbound Message

Instead You can do everything you can do with workflows using process builder.

With process builder, you can also update all child records starting from the parent record, which is not possible with workflows (only vice versa is possible using cross object field updates).

Only thing you can't do with process builder that you can do with Workflow is sending Outbound message without code.However workaround of this limitation by calling apex code from a process. 

Hope it may help you.

Process Builder
The Lightning Process Builder is a new workflow tool that helps you easily automate your business processes by providing a powerful and user-friendly visual representation of your process as you build it. Instead of doing this repetitive work manually, you can configure processes to do it automatically. The Process Builder’s simple and powerful design allows you to automate processes

You can use the Process Builder to perform more actions than with workflow:Create a record
Update any related record
Use a quick action to create a record, update a record, or log a call
Launch a flow
Send an email
Post to Chatter
Submit for approval
Call apex methods
But the process builder doesn’t support outbound messages.

Workflow
Workflow is business logic that evaluates records as they are created and updated and determines if an automated action needs to occur. In a way it allows records to speak up or do something – update data, notify people or external systems.

Workflow does only 4 actionsCreate Task
Update Fiel
Email Alert
Outbound Message

Thanks