A Laptop is a mobile computer with a full-sized keyboard, flip-up monitor and space for built-in optical storage drives.
A Notebook is a mobile computer with a full-sized keyboard and a flip-up monitor. It does not have space for built-in optical or floppy-disk drives, but instead provides communication ports so that an external drive can be connected. This allows a notebook to be much thinner than a laptop.
A Netbook or mini PC is a mobile computer with a keyboard smaller than full size (some are less than half the size) and a flip-up monitor. Generally uses a cut down processor such as the Intel Atom with significantly less power. As with a notebook, it does not have space for built-in storage drives, but does provide ports for connecting an external drive.