It’s browser (not IP). A session is basically data stored on your server that is identified by a session id sent as a cookie to the browser. The browser will send the cookie back containing the session id on all subsequent requests either until the browser is closed or the cookie expires (depending on the expires value that is sent with the cookie header, which you can control from Django with set_expiry).
The server can also expire sessions by basically ignoring the (unexpired) cookie that the browser sends and requiring a new session to be started.
There is a great description on how sessions work here.
What are HTTP sessions?
To display a webpage your browser sends an HTTP request to the server, the server sends back an HTTP response. Each time you click a link on website a new HTTP transacation takes place, i.e. it is not a connection that is persistant over time (like a phone call). Your communication with a website consists of many monolitic HTTP transactions (tens or hundres of phonecalls, each phonecall being a few words).
So how can the server remember information about a user, for instance that a user is logged in (ip addresses are not reliable)? The first time you visit a website, the server creates a random string, and in the HTTP response it asks the browser to create a so called HTTP cookie with that value. A cookie is really just a name (of the cookie) and a value. If you go to a simple session-enabled Django site, the server will ask your browser to set a cookie named ‘sessionid’ with such a random generated value.
The subsequent times your browser will make HTTP requests to that domain, it will include the cookie in the HTTP request.
The server saves these session ids (for django the default is to save in the database) and it saves them together with so called session variables. So based on the session id sent along with an HTTP request it can dig out previously set session variables as well as modify or add session variables. If you delete your cookies (ctrl+shift+delete in Firefox), you will realize that no website remembers you anymore (Gmail, Facebook, Django sites, etc.) and you have to log in again. Most browsers will allow you to disable cookies in general or for specific sites (for privacy reasons) but this means that you can not log into those websites.
Per browser, per window, per tab, per ip?
It is not possible to log into different GMail accounts within the same browser, not even from different windows. But it is possible to log in to one account with Firefox and another with Chrome. So the answer is: per browser. However, it is not always that simple. You can use different profiles in Firefox, and each can keep different cookies and thus you can log into different accounts simultaneously. There are also Firefox plugins for keeping multiple sessions, e.g. MultiFox.
The session all depends on which session cookie your browser sends in it’s HTTP request.
To get the full understanding of what is going on, I recommend installing the FireBug and FireCookie plugins for Firefox. The above screenshots are taken from FireBug’s net panel. FireCookie will give you an overview of when and which cookies are set when you visit a site, and will let you regulate which cookies are allowed.
If there is a server side error, and you have DEBUG=True, then the Django error message will show you information about the HTTP request, including the cookies sent