The E-Ticket can be the printed e-mail confirmation that they sent you or JUST the number. This is sometimes called the 'Locator'. I sometimes just write it down in my diary.
If you check-in online, then you can print your boarding pass 9or not as you choose). On the day, bring the Locator OR the printed boarding pas AND your Photo ID. If not a passport, even for domestic flights, this is needed and new style driving licence will suffice as it has a photo.