Symmetry is an age-old device of the visual artist. The word refers to two halves that perfectly mirror each other. In design, symmetry is closely tied to balance. A perfectly symmetrical design achieves balance and a sense of stability.
There is also asymmetrical design, where the two halves are balanced but do not mirror each other perfectly. The majority of websites have an asymmetrical layout.
Most often we find the logo in the top left, balanced by some navigational elements on the right. In the body, the side columns are usually balanced by either the main content or other columns.
While perfect symmetry is certainly not new to the web, it is gaining momentum. Expanding screen sizes and the proliferation of platforms and viewing options are making it a more attractive option. By mirroring the halves of a design, you not only get a greater sense of balance, but also improve the flow from top to bottom.