Putting up paintings, stringing up lights, and getting some flowers are all ways to decorate a house. When you decorate, you make things look better or at least more interesting.
Even an attractively furnished home can be decorated for a holiday or party. That means you dress up your house to celebrate the special occasion. Decorate is also used to describe certain kinds of success. A famous athlete might decorate a gym with trophies and championship rings. We say a person in the military, like a general, is decorated when he or she is given medals for bravery or other accomplishments.