MYW is the name of the tickets generally being sold at WDW. They are called this as you make the choice of the tickets to get. You choose the number of days and none, any, or all of three options.
A Base ticket is anywhere from one day to ten days. It allows you to go to one of the four major parks (AK, EC, MGM, MK) per day. You can leave and return to the same park. The more days on the length of the ticket the lower the per-day price.
The hopper option allows you to visit more than one major park per day. You can go to one, two, three, or all four parks on the same day and only get charged for one day use. Note that without the hopper option you can only go to one major park per day.
The Water Parks Fun and More (WPF&M) Option will allow you into the five minor parks (BB, DQ, PI {multi-club}, TL, WWS). You will get between three and six entries to the minor parks based on the number of base days. Again leaving and returning to a minor park on the same day does not use up another entry. However, if you go to two minor parks on the same day it will use up a second entry.
All MYW tickets will expire at the close of the 14th day from first use. If first use is on a Sunday, the tickets may be used for park entry until 11:59pm of the second Saturday. At that time any remaining entries (entitlements) are cancelled. There is a No Expire Option which costs more depending on the number of Base days on a ticket. If you get this option the entitlements on the ticket will not expire automatically at the end of the 14th day, but are good until all used up; this includes both Base days as well as WPF&M entries.
Note, any ticket may be upgraded by adding days and/or options at any time until the end of the 14th day. The two exceptions are that you cannot go beyond ten base days and you cannot do any upgrade more than 14 days after first use. You do not have to use the tickets on consecutive days.