2DAY/2PARK Tickets and 3DAY/3PARK Tickets are options for those guests who are not interested in park hopper tickets. Keep in mind, because these tickets are considered discounted ticket media by Walt Disney World, they do have expiration dates (unlike Park Hopper tickets).
If you purchase a 2DAY/2PARK ticket you have 4 days with which to use your two visits before the ticket expires. If you buy the tickets on a Sunday, you could visit Magic Kingdom on Sunday and Disney-MGM Studios on Wednesday. By Thursday the tickets would be expired. The 3DAY/3PARK ticket gives you six days you use your 3 visits. (this includes the day you buy the ticket). Also remember that you cannot visit more than one theme park in a day.
Many guests will begin by buying a 1DAY/1PARK Ticket ($50 plus tax). You may then chose to upgrade this ticket to either the 2DAY/2PARK or the 3DAY/3PARK Ticket. You must upgrade at Guest Relations (at the theme parks) on the day that you use your original ticket.
The savings on these types of tickets amounts to $5 off the second (and third) days. (Example: a 1DAY/1PARK ticket is $50 plus tax. A 2DAY/2PARK ticket is $50 + $45 plus tax. A 3DAY/3PARK ticket is $50 + $45 + $45 plus tax. You receive this savings if you purchase these multiday tickets from the beginning or if you chose to upgrade.
One last thought: All Walt Disney World ticket media is non-refundable. If you are unsure whether you want to visit for more than one day, buy a 1DAY/1PARK ticket and upgrade it at Guest Relations. Because of the expiration date, you will be unable to simply save the unused portion of the ticket for a later visit.
Hope that was helpful and not too confusing

Feel free to ask any questions you may have.