Good description should make a scene vivid to the reader. That means it should be clear, strong, and believable. This applies to both real places and events, or imaginary ones.
When writing descriptively you should consider the time and place. For example, a deserted park at night requires a completely different description from a busy park during the daytime.
How much description you put in depends on the purpose of your writing. If you want to set a scene for a story you will need less than if you are opting for a descriptive essay.
It is possible to put in too much description. You should select details which are needed to create the impression you want your reader to have. Focus on what is needed for plot, to create a mood, or to capture atmosphere.
Consider these two descriptions of a room.
The room was square with a window along one side. It had four chairs and a TV and video. There was a drinks cabinet and computer in the corner. The carpet was red and the ceiling cream.
The room was brightly lit by a large window and housed several modern pieces of electrical equipment but the effect was softened by a drinks cabinet and a warm red carpet.
The first description is like a list. It gives a lot of information, which will not be needed later in the story.
The second gives an overall impression of the room and also manages to hint at the character behind it. In this case, it suggests someone who likes technology but wants to be comfortable too.
Consider how the description of the following scene helps to portray the mood of the character.
A wet, dull day greeted Mary as she stepped into the grey light. Men huddled by in drab wet coats or stood in dismal doorways waiting for a bus which never seemed to arrive.