Georgetown is nice because there are a lot of unique and interesting boutiques there. I like Annie Creamcheese for vintage clothes/shoes/jewelry. Neiman Marcus' Cusp store. Intermix. And there are your typical stores like Betsey Johnson, Diesel, Zara (this one has two or three floors), Sephora, JCrew, Barneys, etc.
Friendship Heights has all your upscale department stores like Bloomies, Lord&Taylor, Saks, and discount stores like Loehmanns, TJMaxx, Filenes, Stein Mart.
U St area has some quirky shops like Junction. Just google "U St DC boutiques" or something like that.
Dupont Circle also has some boutiques, though they're more hit-or-miss like Lou Lou's (accessories), Secondi (consignment). And Metro Center/Chinatown has more typical shopping like Macys, H&M, Zara, Urban Outfitters, etc.
Hope this helps!