If your hair is sleek, it is smooth and glossy. If your car is sleek, it's shiny and looks like it goes fast. Sleek always means smooth, glossy, and streamlined.
Sleek is a variant of the word, slick. When you slick back your hair with grease, it becomes sleek. Cats are often called sleek with their silky coats and lithe movements. A dress could be described as sleek, if it fits tightly to your body and makes you look polished and perfectly fit. When something is sleek, it's aerodynamic.