Rolling memberships have a duration entered, typically 365 days, set in the Duration (Days) field. This means if you purchase the membership on May 15th, it will expire on May 14th the following year. Rolling memberships also have an option to extend them to the last day of the anniversary month. If Expire on the Last Day of the Month is set to YES, then the membership purchased on May 15th will expire on May 31st the following year. 

Monthly Memberships can also be considered a type of rolling membership, which renews at the end of each month automatically.