This update is a significant change for Stardew Valley... Not only does it include a good amount of new content (new items, events, etc.), but it also includes the long-awaited addition of co-op multiplayer!

New Features:

  A traveling festival comes to town For three days in winter, a traveling festival visits Pelican Town. Unlike a normal festival, farmers can come and go as they please, and the location isn't "locked out" at any point (there's no setup). Pelican Town is different for a few days, and most of the townspeople's schedules change to reflect that. The festival offers farmers a once-a-year chance to acquire unique items, purchase an original work of art from the famous Lupini, enjoy some live entertainment, and more....
A winter mystery... and a new collection type In winter, farmers might encounter a strange event... which will ultimately grant them the ability to start a new, "secret" type of collection. This new ability will surely result in unusual new adventures and greater knowledge...
Help someone out in town... Farmers who have made great progress for themselves will have the opportunity to help someone in a significant way... New Character Events There are new character events, some of which grant you permanent bonuses. There are also a couple of "secret" events that you'll only encounter if certain conditions are met.
Skull Cavern Changes

• 3 new monster types added (more powerful versions of familiar enemies)
• "Treasure Rooms" added (rare chance of finding a room with a chest containing useful items... including two that can't be found any other way)
• A new reason to attempt a deep dive...

Ability to change professions If you regret your choice of profession, or simply want to try out something new, there is now a way to change your professions. It comes with a price, however...
Signs Signs are craftable items which are available to you from the very beginning of the game. Any item in Stardew Valley can be "clicked" onto the sign, and it will display that item (the item won't be consumed).
Decorations A variety of new outdoor decorations have been added, including many that are "seasonal", meaning that their appearance changes with each season. Multiplayer makes decorating even more fun.
Auto-Grabber An expensive item that can be placed in a barn and automatically harvests milk and wool from the animals each morning. Garden Pots Garden pots allow you to grow any crop indoors year-round. Think of them like one-tile greenhouses. When placed outside, however, they only grow crops that are in-season.
New Crop There is a new crop that only grows indoors. Fireplaces are now furniture This means you can move your fireplace around, and also buy a couple of different kinds of fireplaces. Hats on horses You can now put hats on your horse. Enjoy... (this was actually an original intended feature in Stardew Valley two years ago, but I never finished it and had completely forgotten about it. Now it's finally finished)
  Now, a more in-depth look at how multiplayer works... Stardew Valley now supports up to 4 player co-op. Co-op is nearly identical to single-player, but with 1-3 other friends playing together with you to achieve a common goal. Any single player game can be "converted" to a co-op game by having Robin build one or more cabins on your farm. One player serves as the host, and the other 1-3 players connect to the host in order to play. Therefore, the host must be in-game at all times when the group wants to play. The new update adds a Co-op button to the title screen. Clicking the button will bring you to the co-op menu, from which you can:

• Host a new co-op game
• Re-host an existing co-op game
• Join a new farm (provided that any of your friends are hosting and have a cabin available for you)
• Re-join a farm (provided that the host is in-game)
• Join a LAN game (by entering the IP address of the host)
• Enter an invite code (generated by the host) to join a game (this allows for Steam/GOG crossplay)

When hosting a new game, you'll have some new options available to you:

• "Starting Cabins" gives you the choice to start a new game with 1-3 cabins pre-positioned on the farm. If you decide to go this route, you'll have two "Cabin Layouts" to choose from: "Nearby", which places the cabins close to one another, encouraging a communal farming style, and "Separate", which places the cabins far apart and allows for more independence.

Alternatively, you can start with no pre-positioned cabins and instead build them yourself (via Robin).

• "Profit Margin" adjusts the profit margin of goods that you sell. You can choose between "Normal" (the original Stardew Valley amount), "75%", "50%", and "25%". This is, in effect, a way to increase the difficulty of the game. For larger groups of experienced players, it might be desired. However, keep in mind that there already is some "scaling" of difficulty built into the game, since each player will need to upgrade their own tools, upgrade their own houses, buy bigger packs, etc.

In co-op, all players share the same pool of money, and are working together on the same farm. The state of the game world, including the main storyline, is shared between all players. However, each player has their own private inventory,  skills, achievements, collections, relationships, quests, and crafting/cooking recipes. When it's time to go to bed, all players must enter their beds. Only then will the day end. Players can marry each other by crafting a "wedding ring" (An old Zuzu City tradition), and offering it to another player. If the other player accepts the ring, the couple is now engaged. The chat box allows you to communicate with each other... with the help of nearly 200 custom Stardew Valley emojis!
  Here's a trailer: https://youtu.be/-vlBE3vSfNU   I hope you enjoy the new update! -Ape