What is the order of execution of workflow, process builder and triggers?

Below is the Salesforce Order of Execution:

• System Validation rule (required field, field format) (SV)
• Before Triggers are executed (BT)
• Custom Validation rules are checked (CV)
• After Triggers are executed (AT)
• Assignment Rules are executed (AR)
• Auto-Response Rules are executed (ARR)
• Workflow Rules are executed (WR)
• Before and after triggers are executed one more time if the workflow rule updates a field (BT & AT)
• Escalation Rules are executed (ER)
• Parent Rollup Summary Formula or Cross Object Formula fields are updated in the respective objects. (RSF, COF) (These parent records also goes through the entire execution order)
• Criteria Based Sharing rules are evaluated (CBS)
• Any Post-Commit Logic is executed (PCL)  (like sending an email)

Below will help you to remember Order of Execution in Salesforce:

SV -> BT -> CV -> AT -> AR -> ARR -> WR (BT, AT) -> ER -> RSF, COF -> CBS -> PCL