You can detach the keyboard of your device and use the screen as a tablet. Detaching the keyboard makes the device lighter and gives you the possibility to switch easily between laptop-mode and tablet-mode.
Contrast ratio is the visual distance between the lightest and the darkest colour that may be reproduced on the display. A high contrast ratio is desired, resulting in richer dark colours and more distinctive color gradation.
More memory slots (also known as DIMM slots) allow you to add more RAM to your computer. It is also useful when upgrading, as you can add RAM to an empty slot instead of replacing an existing memory module.