"Instant Download" tickets will be ready to be downloaded soon after the seller confirms your order, while "Email Delivery" will be ready at a later date. Keep in mind, you are guaranteed to receive your tickets in time for your event, regardless of which delivery method you choose.
Both Email Delivery and Instant Download ticket types will be in a PDF file for you to download and print. You will then need to print out the tickets, bring them to the venue, and they will scan the barcode for entry.
"Mobile E-tickets" are a little different in that the tickets must be shown from a cell phone, not printed on paper.