While writing certainly has its limitations as a medium for expression, descriptive writing has the capability of recreating scenes through precise, verbal accounts of things and places. The phrase "a picture is worth a thousand words" exists for a reason: when we can see or imagine something visually, we are better able to place ourselves within that situation or moment. When you are writing descriptively, you should provide telling details about the appearance of the place, person or thing you are trying to bring to life so that your audience can similarly "see" it.
Descriptive writing should include other details from your five senses in addition to visual ones. In order to show your reader what a place or time or situation was like, explain to them how this place smelled or sounded. Recount what this thing or person you are writing about felt like to the touch. These details will, like visual imagery, help your reader to understand what the subject of your writing is like on a visceral and personal level.
Another important effect to strive for in descriptive writing is a sense of intimacy with the place, person or object. While you may know your subject very well, remember that your reader does not. Therefore you need to close the distance between the reader and the subject as much as possible. One way to create a sense of immediacy is to place your situation in the present tense. That way, instead of experiencing the scenario or characters through the foggy veil of the past, your audience will be seeing and hearing them as you did originally, in the "now."
It is important, in descriptive writing as well as in other types of writing, to know what to leave out, and to include only the most notable details of your subject. Rather than listing twenty ways that the character of your piece looked when she smiled, describe as accurately as you can the one smile you remember most. Being selective with your details and paring down excess modifiers and phrases will not only streamline your points, but will also give your reader a definite picture of the place or character instead of a somewhat nebulous, generalized sense.