Tablet is self explanatory - keyboard is either missing or virtual -- or it's a laptop where the screen twists right round so the device can be held as a tablet.
Netbook -- generally smaller format with long battery life -- 10 inch screen, no optical drive, maybe solid state main drive, wifi and/or cellphone internet access.
Laptop -- the default, though these days often used as a desktop substitute.
Notebook -- difference between this and laptop vague. Probably down to smaller size.