Event Collectibles were introduced with the third Chest Hunt Event, Discover the Mystical Eggs. They are similar to Event Treasures in that there are multiple types, but for the collectibles, you simply get rewards at set levels after collecting a certain amount of them, whereas Event Treasures required you to complete a set of 6 different types and usually 1 of the 6 types has a much lower drop rate than the other 5.
12/16/2013 - 10/25/2013 (23:59 EST) The Blessing of the Light - Starlight Bells
10/11/2013 - 10/17/2013 (23:59 EDT) The Vermillion Summoning - Sigil Tablets
8/23/2013 - 8/29/2013 (23:59 EDT) The Solstice of the Revenants - Warding Lanterns
7/04/2013 - 7/10/2013 (23:59 EDT) The Archive of Forsaken Chronicles - Ancient Books
5/02/2013 (6:00 EDT) - 5/09/2013 (23:59 EDT) The Ancient Temple of Memories - Jewelled Fossils
3/29/2013 - 4/7/2013 (23:59 EDT) Discover the Mystical Eggs - Jewelled Eggs