A desktop PC (Personal Computer) is designed to stay in the same place all the time. Desktop computers are generally much faster and have higher performance than Laptop computers
A Laptop is a simplified Desktop. They are very portable and can be held in one hand. Typical specs for a mid range laptop would include, 750GB HDD, 6GB RAM, 2.4Ghz Dual core processor.
Notebooks are basically the same as Laptops but are smaller and usually have no DVD drive.
Netbooks are the smallest of the gang and are primarily designed for internet use and therefore don't have particularly good specs, perhaps 1GB RAM, a 120GB HDD and a single core 2.1Ghz processor.