What is the difference between a Billing Run and Payment Run?

In a nutshell, Billing Runs allow for the automatic generation of invoices while Payment Runs allow for the automatic collection of payments for invoices due. 

Billing Run

Billing Runs determine all unbilled orders and billing schedules currently in the system; then creates, and communicates corresponding invoices for each customer. Billing Runs can be manually started or scheduled to occur at a variety of intervals. Explore additional Billing Run articles. 

Payment Run

Similar to Billing Runs in concept, but for payments. Payment Runs apply a payment to all invoices that have a balance due and for the customers; (when Auto-Pay is enabled and a valid electronic payment method is stored). A Payment Run can be ad-hoc or scheduled to run immediately after a Billing Run or based on a schedule. Explore additional Payment Run articles.