Revenues are the receipts from sales. As well as costs there are several types:
Total revenue is calculated simply - quantity sold multiplied by the given price.
Average revenue is a revenue per unit.
Marginal revenue is an addition to total revenue from one additional sale - one more passenger, for example.
MR will always be longer than AR because the firm can sell more by reducing the price of all units.
The total revenue curve has an upside-down U-shape, because it indicates how customers react to different prices due to variations in PED. The total revenue is maximised where the PED = 1.