A traveller is someone who goes from one place to another. This could be from one city to another or from one country to another. An individual moving about the city in which he lives cannot be called a traveller. A traveller can get from one place to another by different means — an aeroplane, a ship, a car, a bullock cart, a horse, etc. The traveller may even choose to walk from one place to another all alone; after all, that’s how a lot of people travelled in the old days.
A passenger, on the other hand, is someone who is never alone; he always travels in the company of others. Furthermore, a passenger never walks; he is always in something that is manmade — car, plane, train, ship, cart, etc. Two people sitting on a horse cannot be called passengers; they are both riders. But two people sitting in a coach being pulled by horses can be called `passengers’. When you say that someone is the passenger, what it implies is that the individual is not the driver. You can be a passenger in your hometown.