It lives and dies with your browser session and is not shared between tabs. It doesn't expire automatically. So if you never close your browser it never expires.

So when the tab/window is closed the data is lost.

Each sessionstorage area is allowed 5MB of storage (in some browsers 10MB). Whereas cookies only allow 4kb (or more in some browsers). Cookies however have a set expiration date.

As Christophe wrote in the comments, localstorage never expires. It's also shared across tabs and is the same size as sessionstorage (5MB).