From this page of our Valheim guide, you'll learn what editions of the game you can buy and the contents of all of them.
This page is under construction and will be updated with additional editions as soon as they appear.
At the moment of writing this guide, Valheim is available in Steam Early Access. This means that the game is not finished yet, and its premiere is announced for the year 2022.
By deciding to buy Valheim in Early Access, you receive instant access to the game, but new content will be added continually until its real premiere. Moreover, all players playing the game in Early Access have the ability to send their feelings and thoughts about it, and the authors may update the contents following received suggestions.