With the Legacy of the Void, the final StarCraft 2 expansion, which came out almost five years ago (November 2015), Blizzard introduced an entirely new way of playing called Co-Op Missions.
This mode allows you to play specially designed missions, crafted for a co-op experience. Thematically and in gameplay terms, these missions are similar to those from the singleplayer campaign, but they offer more challenges and customized ways of playing them. This mode quickly became the most popular StarCraft mode of playing by far, and it remains so to this day, because it offers fun and exciting experience with countless strategies and combinations, all of which you can enjoy online or with your friends.
You choose one from currently 18 available commanders and play one of the currently 15 available missions, divided into three categories: Assult, Control, Siege. It's worth noting that Blizzard is continuously adding new Commanders and Missions.
Each co-op commander offers a deep and unique gameplay style, with a unique set of units, building, upgrades, abilities, etc. As you level up your commanders, with experience you get from playing missions, you unlock more units and upgrades, which makes your commanders stronger, but also unlocks harder levels of play. There are also "Weekly Mutations," which are special weekly modifications designed to add different new challenges for players who want more.
StarCraft 2 is free to play, and so is this mode. But Commanders are free to play only up to level 5, and then you need to buy them if you want to continue playing them. However, starting this weekend, all co-op commanders will be completely free to play through Thursday, April 2. This means that all of them will have unlocked all units, upgrades, and other stuff that you usually get by leveling up, in order to allow players to experience them properly.
Additionally, everyone gets 100% Co-op bonus XP through Thursday, April 16, which means that you can level up your commanders faster to level 15, and have stuff unlocked even after this promo ends.
As a side note, we mentioned that basic StarCraft 2 is free to play in general (not just now), which means you can play Wings of Liberty singleplayer campaign, Ranked and Unranked games against other players, Custom games and Arcade games.