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.
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.
Thunderbolt is a connection that allows you to attach external peripherals to a computer, and can carry both data and display signals. Using a single port you can connect up to six devices through a hub or daisy chain configuration.