The device has a display with three full subpixels per pixel, resulting in a sharp and crisp picture. Pixels in some displays (like AMOLED) share one subpixel to preserve space. This can result in a less crisp, slightly blurred image.
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.
Retina display refers to a high-resolution screen technology developed by Apple. A Retina display has a high pixel density, with more than 300PPI, which makes it impossible to spot pixels from a normal viewing distance (10-12” from the eyes).