Reign of Giants was released in April last year, and brought two new characters, spring and summer seasons (spring sees the flora and fauna become more active and aggressive, while summer causes heatwaves), and various new enemies including the titular giants to the game. The first new character, Wigfrid, is a stage actress who thinks she's a valkyrie, only eats meat and loves fighting, while Webber is a young boy who lives inside a spider. Naturally.
Today's update adds all that to the co-op Don’t Starve Together, with a few minor changes, which you can readThere's are also a few general changes, including the requirement to feed your animals unless you want them dying on you, the ability to burn wooden structures, and a reworked system that causes you to lose sanity if you stand out in the rain too long.
Klei says that you should be able to use your save games with the new patch, though you'll only gain access to the core mechanics. To see the new Reign of Giants content, you'll need to start a new world. Don't Starve Together is still in Early Access. Owners of the core game will get hold of it for free when it's ready to release in full.