what is the difference between 'order' and 'order line'? I was trying to understand degenerate dimensions. But I quite often encounters the term "order line" as grain in an transaction table. Could someone please explain the difference between the order and order-line ?
Generally, an order contains information about the order.  Who placed the order, what time they placed it, the shipping address associated, the billing address, payment method, when it was fulfilled, etc.  It often does not contain any information about what was ordered.

The order line generally contains information about what was ordered, this is done because a single order can have multiple items in it.  So the order line would specify the item ordered, the quantity ordered and the price charged, and there would be one line for each different item ordered.