In essence you have an assembly line and you can only move cakes forward and backwards on that line. Above each station is a machine. It can eject a cake, add on icing, add on decorations, and so on. You have to put the cake you're working on beneath the right station to get it to be worked on.
You can't just build ANY cake though - you are getting orders in from customers. Maybe somebody wants a three tier cake where the bottom level is strawberry, the middle level is chocolate and the top level is vanilla. This means you have to slide the cake forward and backwards to get each of those items done.
It's not that challenging when you're working on just one cake - but once you get several cakes all on the conveyor belt at once, the challenge begins!
So if you look at this screen -

An order is for a three tier cake with a shamrock, as shown in the top left. I have made that cake. I started by putting the cake beneath the "circular pan" spot and clicking. Then I moved the cake over to the "pink batter" spot and clicking. By moving the cake left and right, and selecting the appropriate frosting and decoration, the cake got made.
The trick comes when you have multiple cakes going on the same conveyor belt. You have to try to move the cakes left and right and work on them all at the appropriate time without getting things confused!
This is a great logic puzzle for all ages, and builds real world figuring-out skills. Plus, it comes free with Windows Vista, which is a great price!
This is the advanced level of the game - you start out much easier :) So you work your way up to the challenge.
Highly recommended as a daily exercise to keep those grey cells active.
NOTE: I do not think you can buy this game separately. It comes free with Vista, so if you're interested in this game, it might be time to upgrade to Vista!