The basic principles of design are true for Web design as well as other design. They show you how to put together design elements in an effective manner. Design is more than just slapping HTML tags up onto a page, and using these principles will help you build more pleasing and useful designs.
Balance is the distribution of heavy and light elements on the page. Larger, darker elements appear heavier in the design than smaller, lighter elements. The principle of balance shows you how to lay out your pages so that they work.
When most people think of contrast, they typically think of colors or black and white. But there is more to contrast than color. You can have contrasting shapes (square vs. circle), or contrasting sizes (large vs. small), or contrasting textures (smooth vs. rough).
Emphasis is what the eye is drawn to in a design. It's tempting to give everything equal emphasis or try to emphasize everything in a design, but this ends up making the design bland and flat. Instead, as a designer you should determine the hierarchy of the page and then apply the emphasis to the elements based on that hierarchy.
Rhythm is also called repetition. Rhythm brings an internal consistency to your Web designs. Patterns are easy for humans to comprehend, and repetition provides patterns that make your site easier to comprehend.
Unity is also called proximity. It is the principle of keeping like elements together and diverse elements further apart. Unity pulls elements together.