What is the difference between Process builder and workflow?

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 does only 4 actions

• Create Task
• Update Fiel
• Email Alert
• Outbound Message

More information here

Workflow

Workflow enables you to set up workflow rules that identify what kinds of record changes or additions trigger specified workflow actions, such as sending email alerts and updating record fields.

Visual Workflow

Visual Workflow enables you to create flows, which are applications that are user-triggered instead of event-triggered. 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.

Process Builder

Process Builder can be utilized to automate all workflow actions (with the exception of outbound messages). Traditional workflow can still be utilized in parallel, however, Process Builder gives you the ability to do things that workflow can’t. Although, somewhat of a double edged sword this puts some very powerful tools at the disposal of an experienced administrator.