Actually, neither answer is quite correct. Here it is from the docs:
Given a year with 2 digits, returns a year in the next century if the year is <50 and the last 2 digits of the current year are >=50; returns a year in the preceding century if the year is >=50 and the last 2 digits of the current year are <50.
Round year. Accepts either 4-digit or 2-digit input. If 2-digit, provides the same return as RR. If you don't want this functionality, simply enter the 4-digit year.
4-digit year; "S" prefixes BC dates with "-".
Last 3, 2, or 1 digit(s) of year.
So YY will assume that you are talking about this century and will insert 2095 when 95 is entered, and RR will follow the rule that >50 means last century and insert 1995. This is as rmgirma said. However, if you enter 35, both YY and RR will insert 2035. An important distinction.