Best answer: At its heart the Steam Deck is simply a handheld PC and while it runs a Linux-based OS out of the box you will be able to replace that with Windows if you wish.
Steam Deck is just a PC and it can run Windows
Linux PCs run on the same hardware as Windows PCs so in essence, the Steam Deck is just a PC. A very small, gaming-optimized PC, but still a PC nonetheless. And that means a certain freedom to tinker with the software, and yes, install Windows 10 or Windows 11 if you wish.
Valve recommends not wiping SteamOS for Windows — naturally — but designer Lawrence Yang told IGN that it was possible.
"We don't think people should be locked into a certain direction or a certain set of software that they can install. If you buy a Steam Deck, it's a PC. You can install whatever you want on it, you can attach any peripherals you want to it. Maybe a better way to think about it is that it's a small PC with a controller attached as opposed to a gaming console."
What you would lose by installing Windows is all of the UI optimizations that Valve has designed with SteamOS to make it a good experience on a handheld. But you're going to be free to do as you wish with it. With 16GB of RAM onboard and a custom quad-core AMD APU, it'll certainly have enough horsepower for Windows.
You don't need Windows on Steam Deck to play your games
Hearing "Linux" and "gaming" in the same sentence might cause some concern that they won't be able to play their favorite games. Indeed, while there are a lot of great titles natively available on Linux, such as Valheim, Windows is still the preferred platform. But it's a total myth that Linux means no compatible games.
The Steam Deck runs SteamOS and uses a tool called Proton. This is a compatibility layer that translates Windows games into something Linux can understand when you consider it at its most basic level. Our full explainer will give you more detail but needless to say, you shouldn't worry. Valve has been developing Proton for years now and it's very good. It's not perfect, but it has the biggest cheese in PC gaming behind it.
You should also — to some degree of success — be able to play games from sources like Epic Games, GOG, Ubisoft Connect, and Origin. There are already tools on Linux to integrate these platforms and there's no real reason they shouldn't also work on the Steam Deck.
What won't work is anti-cheat, the Linux gamers arch-nemesis. So Apex Legends, PUBG, Destiny 2, and the like are right out, at least for now. Maybe the Steam Deck can help Valve give the developers of these pieces of software a little nudge.
Bluetooth controllers, keyboard, and mouse
As it's a PC there will also be a good selection of accessories that will work with the Steam Deck. Bluetooth input devices such as controllers, keyboards, and mice will work, and Valve is planning to sell a dock as well that will help you hooked up wired accessories.
Connected to an external display with a controller hooked up the Steam Deck becomes a full games console, akin to the Nintendo Switch when its docked.