Simply put the sid is a session id tracker to ensure you are logged in, your UA hasn't changed, your ip is still the same, etc, etc. Its benign and if you use the link including the sid, no people won't be logged in as you, it will just make it easier when you are traversing the site that you don't get logged out, that's all. For example, if you are logged into the site, your ip changes (happens with ISP dynamic ip renews) it will log you out. If you change you UA mid-session, it will log you out, if you paste into another browser that doesn't have the logged in cookie information, it will prompt you to log in. So on. Its a way for the system to manage client/server requests with some degree of intelligence to make sure delivery, that's all.
ADD: Say I give you a link with MY sid in it RIGHT NOW, and you are also logged into the system (or not for that matter) and you use it, it will replace all subsequent requests with YOUR sid instead of the one I gave you. Its a way to "track" for lack of a better word, your requests within the session, that's it.
Links:
Developmental usage:
http://wiki.phpbb.com/Function.append_sid
Another explanation of your question:
http://www.phpbb.com/community/viewtopi ... &t=2092992
.)