Greenland, like most other European countries, has 220-volt AC, 50Hz current and uses two-pin continental plugs. If you visit from the UK and Ireland, you will need an adaptor for electric appliances, whereas North Americans need a transformer in order to use their 110/125V appliances.
Some appliances, like chargers for laptops, digital cameras or mobile phones, may already be compatible with multiple voltages and may just need a travel adapter.
Greenland uses the standard Europlug socket with two round prongs so you can use adapter types "C" or "F" (often labelled as a Northern Europe adapter). Make sure to check the voltage labels on your appliances or chargers to see if you need to use an adapter or converter+adapter.