Your Outlook data is usually (for a default, stand-alone configuration) stored in a .pst file (often Outlook.pst) buried on your hard drive. Here are a couple of ways to do what you want:
1) Move your desktop .pst file to a USB memory stick (you may want to search for instructions on how to do that, as I don't have time to write them here), and tell Outlook on your notebook to use that same file. Plug your USB memory stick into whichever computer you want to use Outlook on, and your Outlook data will always be consistent. Don't forget to back up the contents of your memory stick on a regular basis (search for instructions on how to do this automatically, if using Win2K or WinXP). Pitfalls: try not to forget your memory stick, and look into encryption settings in case you lose it.
2) Move the .pst file from your desktop to a folder on the notebook that is shared on the network. Map that drive on your desktop and tell Outlook on both computers to use that file. Pitfalls: notebook must be on the network to use Outlook on the desktop, and performance may suffer when accessing the .pst file over the network from the desktop, especially if you have tens of thousands of items.
There are probably other methods. I used to use the USB memory key and it worked very well. Now though, I only use my notebook (tablet, actually) for Outlook. Even when I'm using my desktop for doing other work, I have my notebook set up right beside it with Outlook open.