You might've noticed the first two pieces at launch last week: A Temerian Armor set and a collection of beards/hairstyles for Geralt. Two more pieces go live this week, adding an alternative look for Yennefer and a new contract in Skellige called "Missing Miners."
Those four are the only pieces of DLC that we knew about ahead of launch. The other twelve are a mystery, and will be released weekly throughout June and July.
The only catch You have to remember to take advantage. GOG Galaxy and Steam users have it slightly easier, since you can manage your DLC in-client. Those who downloaded a DRM-free copy of The Witcher 3 however will need to download and install the DLC each week as it's added. Like patches, it's easy to forget if you're used to an auto-updating client.
And speaking of patches, a new Witcher 3 update went live yesterday. Patch 1.04 adds full key-rebinding, improves texture rendering on High/Ultra, "improves" HairWorks performance (I didn't notice a difference), and thankfully fixes issues related to Alt-Tabbing, amongst other tweaks. You can find the full list of patch notes here.