An e-ticket is an 'electronic ticket' that replaces the need for paper tickets. It makes confirming your flight quicker and easier as all your booking details are stored in the airline's computer system and retrieved using the e-ticket reference given to you when you book.
You can never lose an e-ticket and, with most airlines, all you need to do is quote the reference or show your passport in order to check in.
The airline will issue you with a boarding card at the airport check in desk. Note that some airlines insist that you print the e-ticket confirmation and/or check in online before arriving at the airport. This often applies to low cost carriers..