Well, let's take a shopping cart, for example.
John Doe goes to your site. You sell t-shirts. T-shirt options are:
1) size (s,m,l.xl)
2) color( red,orange,yellow,green,blue,purple)
3) iron-on pattern (space shuttle, horse, Porsche)
4) pattern location (front,back,both)
5) quantity
6) embroidered name
Now, if John wants a single shirt, cookies would probably work well. But, if John is a member of a Porsche collectors club and he is placing an order for all 500 club members, you can assume that there will be more than 2 or 3 different option combinations: small red Porche front Jim, extra large green Porsche back Fred, etc.
Unless you want John to place an different order for every 3 shirts or so, it would be best to use sessions. You could store all 500 shirt orders in a session and not have to worry about the several kB cookie size limitation since the cart information is stored on the server in the session instead of on the client in a cookie.
Each shirt order could easily be 200 bytes or more, which means you'd have a 10 shirt limit (assuming a 2kB cookie size limit) if there is no other information stored in the cookie at all.
The cookie simply stores the session ID, which tells the server which session is in use by this browser.
Does this help get your mind around sessions?
If not, let me know and I'll try to explain it another way.
-Rich
