Rodinia is a small square world (4000 x 4000) that was originally created as a test world for the update to 1.8. It was the first place to acquire the new stone types (andesite, diorite, and granite), among other new blocks. Once the update was ready, Rodinia was linked to the main network.
Rodinia is used by some as a "base" of sorts for people to store their items. This is why it's sometimes referred to as an "ender chest".
The spawn for Rodinia is at Point Zero.