Each space is a group of tabs that a user wants to organize together.
Spaces help users stay focused by separating tabs into groups and only showing what is relevant to the task at hand. These spaces can also be shared with others to synchronously or asynchronously collaborate on tasks together. The space is derivative of a window.
We will add the least amount of features possible to allow the user to manage windows, keep tabs groups congruent to the user's optimizations (productivity, organization, context, etc), and allow them to work with others within these windows. If done correctly, the user won't notice any new clutter, buttons, or overlays, and the window should behave congruently to how they expect it to work today.
History of the windowing system
Windowing is first evident in the first real GUI browser, ViolaWWW, created by Pei Wei. It was a mechanism to allow a user to have multiple sites open at once, without the explicit introduction of tabs (came to ViolaWWW by 1993).
The concept of a window is mostly managed by the OS and most of the interaction by the browser is limited to generating a new window, or a blank set of tabs. The sizing, shaping, and utility of the rest of the window is the same as any basic desktop application.
Unfortunately, most users don't use multiple windows, with the exception of splitting the plane. If we extend the idea that how users manage windows reflects on how many tabs a user has at any point in time, we need to make the windows much more interoperable and flexible. Users should be able to switch between windows (retroactively and proactively) based on their own design. This should make them feel focused, less cluttered, and for thinner clients dramatically improve the performance of their browser.
As the internet has evolved, much of what we do isn't single player. Spaces (windows) will be able to be used by multiple people, where people can hop into the same window, share tabs seamlessly, and look at single tabs while the DOM is synchronized. The idea is to give the window the feel of a google doc. If done correctly, this feature will be extended from watching youtube videos and solving puzzles with a Significant Other all the way to many people making recruiting decisions.