The Steam Deck is a portable, handheld PC that will ship with Valve's own SteamOS (built on top of Linux). The device is billed as an entirely open platform, letting players install games outside of the Steam ecosystem, run additional PC software alongside them, or just replace the entire OS with Windows if they like. Valve is hoping that most (not all) games will work just fine on SteamOS when the Steam Deck launches, with a lot of work going into improving Proton--a compatibility layer that allows Windows games to run on Linux without native support, and without (too many) issues.
The handheld looks like a beefier Nintendo Switch, with two analog sticks and two arrays of face buttons on each side. Below the thumbsticks are two capacitive touchpads that can also be used for directional input, like the two touchpads used on Valve's discontinued Steam Controller. The screen is a 1200x800 seven-inch display, which should be great for running games below 1080p while maintaining crisp image quality.
The Steam Deck features a USB-C port on the bottom for charging, with Valve estimating that the device will last between 7-8 hours on a single charge (depending on the games you're playing). The Steam Deck can also be docked and connected to an external display for higher resolution gaming, but the internals are very much designed for lower resolution play (4K output is at least supported, but don't expect to game at that resolution).
Internal storage is dependent on which version of the Steam Deck you have ordered, but every model also supports expansion via MicroSD cards. The device comes with SteamOS and Steam pre-installed, with support for all of Steam's features such as cloud saves (which will seamlessly sync between the Steam Deck and your PC), messaging, and more. Alternative game launchers, such as the Epic Games Store and Battle.net, can also be installed, either on the standard Linux operating system or whichever one you choose to overwrite it with.