Monthly Collections are items that can be bought from the Cash Shop each month for 250 CSC containing ten items sharing a common theme. Between May 2006 and December 2008 they were known as Donation Presents, since January 2009 they have been called Collections. Shared themes are an addition as of January 2012, before then collection had a few items that were themed on that particular month.
When opening a collection you choose which of the ten items you want. When opening a Donation Present you are given a random item.
Previously, donation items did not distinguish between each month. When the month was over, and unopened present automatically turned into the new month's collection. So a donation present purchased in July 2005 but opened January 2006 would be a January 2006 item.
This practice changed in June of 2006 with the introduction of individual art for each month. All unopened packages sold between July 2005 and May 2006 now contain only items from May 2006.