E-ticket / M-ticket

• What is an e-ticket ?
• What is an m-ticket ?
• How to print and store my e-tickets ?
• Why isn't the option to print-at-home available for all events?
• I wish to give my e-ticket / m-ticket : will the new holder be allowed to enter the venue ?
• All e-tickets / m-tickets of my order bear my name. Yet, I plan to attend the show with other people, will they be able to enter ?