When you buy a plane ticket online from a scheduled airline you are often provided with an 'e-ticket'.
This is a scrap of paper with a reference number on it, which the airline check in staff can look up and confirm that you should be on the flight.
We don't like e-tickets, as they have a nil value whereas a real ticket with coupons can be exchanged for carriage on another carrier if you have problems.
Even carriers don't always accept e-tickets they issue - as was witnessed during several BA computer failures, when passengers carrying real tickets were boarded without question whilst e-tickets passengers waited for BA staff to pour through mountains of printouts.
The e-ticket system depends on trust - passengers trust that a scrap of paper will result in transportation.
Unfortunately, at the moment there are too many situations where a real ticket is much better than an e-ticket.
Where possible, always opt for a real ticket. If it costs a few pounds more, we think it's worth the premium.