**sales_order_place_after:** This event is dispatched after the order payment is placed.

    $order = $observer->getEvent()->getOrder();
    $incrementId = $order->getIncrementId();

**sales_order_save_after:**
This event is called every time the order is saved. For eg. Order status changed, invoiced, shipped & credit memo.

    $order = $observer->getEvent()->getOrder();
    $orderId = $order->getId();