Execution Mode for Process Builder and Flow

Process Builder and Flow Execution Mode

Process Builder runs in the system mode so the object and field level permissions both will be ignored for the user who triggers the Process.
However, if a Process is launching a Flow (which runs in system mode), the whole automation will run in the system mode.

Process Builder like Workflows runs in the System Mode hence not restricted by Field Level Security or object level permissions
Flows run in User Mode, Flows when Launched using a Process builder will run in the System mode.

for more detials:

http://theblogreaders.com/difference-salesforce-workflow-rule-process-builder/

http://theblogreaders.com/difference-system-mode-user-mode-salesforce/

Difference between Salesforce Process Builder and Workflow

Difference between Salesforce Process Builder and Workflow

Workflows Rules allow you to define the following actions:

• Field Updates
• Email Alerts
• Create Tasks
• Send an Outbound Message

Process Builder now allows Admins to do the following actions to compare the workflow rules:

• Create Records (not just task records)
• Update fields on any related record
• Post to Chatter
• Invoke other processes
• Launch a Flow
• Submit a record for Approval
• Call/invoke an Apex class

Process Builder vs Workflow

Workflow actions

• Email Alerts
• Tasks
• Field Updates
• Outbound Messages