Here, you see Hilary Duff walking down the street in a cute and casual look. Now, although there are many trends in this look, the main part is what's classic. In this outfit, the classic pieces are the plain white casual/slightly dressy shirt and the dark wash skinny jeans. These pieces are in neutral/safe colors (white, black, anything dark, gray, brown, khaki, nude..) and they are classic simple styles. The trends in this outfit are the bright yellow heels, the green/yellow hoodie, the scarf and the big black bag. These trends help keep her look updated and chic, and also makes a look (which could have looked slightly dressy) casual and comfortable. The trends also keep the look up-to-date. Without these trends, this look would be quite boring and not as chic and fashionable as it looks now because it would not have anything current in it.
You can dissect almost any fashionable and chic look and separate the trends from the classics. Any fashionable person can also do this with their own wardrobe.