Notebooks are real PCs and in some cases even a real desktop replacement. But there are several major differences. Everything inside a notebook is tailored directly to that model. When you want to upgrade or change the VGA card or the motherboard of your desktop computer, you can get replacement products from different vendors that will fit into your desktop computer. Notebooks have only very limited upgrade options. MiniPCI, MDC as well as PC Cards are standardized and provide some flexibility.
The battery makes the real difference from the desktop. Battery chemistry and capacity vary from model to model. Some notebooks support two battery packs, but this feature will add about 1 pound to the weight of the notebook. Mobile 3D graphics are one the rise, but about 1-2 years behind the performance of a desktop computer.
I (Uwe von der Weyden) have been a heavy user of notebooks for several years and models now. My desktop computer is running as a router and a place to store data. The advantages of the flexibility, the mobility and the TFT display let me forget the disadvantages like the slow HDD, the limited 3D performance and the high price of the notebook.
The notebook is much more of a technical challenge than a desktop computer. We at Tom's Hardware take the gauntlet and will explain everything about a notebook. We will start with a series of Tom's Hardware Style notebook tests very soon.