session is something you set when a visitor enters your page and stays same until your visitor leaves your web page(or you change it). It is basically used to track user behavior. It is key point for user logins. When your visitor logins with his/her username and password, you assign a value into the session and through that value you know that the user is logged, while your user crawls between pages in your site. Otherwise you 'd ask username/password at every page.
Yep, basically what yleiko said. Here's an example of it in use. (in your login script after you authorize the user) $_SESSION['username'] = 'Fred'; (then on your welcome page) echo "Welcome, " . $_SESSION['username'];