when you log onto a site you can let the site put a
cookie on your pc which caches your login info so you don't need to enter it every time you login. if you log onto the pc with the same user name you use the same profile settings which includes information like desktop settings, file locations, cookies and so on.
so if you use your pc logged on as yourself, then someone else comes along and uses your pc with the same logon then as far as the rest of the world is concerned it is you at the keyboard.
if you log out of a specific website then you can login a new account which will update your cookie and you will always be that account until you logout and login with new credentials.
if you expect other people to use your machine and don't want them to go around checking your browsing history (heh) or getting automatically logged into sites you frequent then your best bet is to make a new user account for them which will create a new profile / cookie storage etc. so when you are done just log out, then let them log in and do what they need to do.
there's also considerations like IP addresses which is like your street address on the internet, so even if you have 5 pc's in your home if you are all going through the 1 internet connection then you all have the same internet street address, so if your douchebag brother gets banned from a site you both troll then you'll get banned too. go punch him for it.
you can set various security levels in your internet browser to block cookies and so on, but the tighter the security you set the more of a punish it is to use the net effectively so you have a tradeoff, tighter security at the cost of being blocked / have to click YES any time you want to go somewhere.