And if you want to send it, or interact with server data, you can write that as a script service that is also accessible from Javascript. In limited cases you can also set the value of a field (via Javascript) that the server can pick up on postback.
If you can specify the framework and maybe some more details someone might be better able to answer this for you.
A lot depends on the framework you're using (PHP, ASP.Net, etc).
In the ASP.Net world you can access the browser data via Javascript:
http://www.w3schools.com/html/html5_webstorage.asp
And if you want to send it, or interact with server data, you can write that as a script service that is also accessible from Javascript. In limited cases you can also set the value of a field (via Javascript) that the server can pick up on postback.
If you can specify the framework and maybe some more details someone might be better able to answer this for you.