what is the difference between net books, note books and laptops in terms of their storage capacity?
Netbooks use smaller hard drives, biggest I have seen in a netbook is 250gb (Netbook usually means around 10 inch screen)
Laptops use regular laptop hard drives, biggest I have seen in a laptop is 500gb (Notebooks and Laptops really are the same thing, its such a large category that its hard to pinpoint it.)
The differences in terms of storage capacity are only relative to what capacity hard drive is installed in the machine. The main differences between netbooks and notebooks (laptop is not used any more because long term use in your lap is a Bad Thing) are in physical dimensions, upgrade options, and built in ports. As for drive capacity, that depends on what the drive controller and BIOS will support. If you were to buy a netbook with a 2.5" hard drive, and a notebook with a 2.5" hard drive, theoretically you could buy 2 of the same capacity drives and put one in each system.
The main difference between the 3 notebooks I have and the netbook I acquired last year are physical size (the netbook has a 9" screen) and available ports. The lack of a serial, parallel, or docking port on my netbook don't concern me since all my current peripherals are USB connections. I haven't purchased a serial or parallel attached device in quite some time. The netbook is also about the size and weight of a large paperback book. It's great to throw in a backpack and take everywhere. If you decide on a netbook, I would advise you find one that meets all your requirements, then ask if they have a 'long life' battery. My netbook came with a 6-cell battery, however they make a 9-cell battery for it that provides (per the manufacturer) an additional 2 hrs work time between recharging.
Either way, enjoy your purchase, make the most out of it, and always do your research.