WordPress websites are all built with a theme of some sort on the backend, which is the code to provide the layout and design of the site. Many people choose wordpress for their website purely because there is the capacity to have one of many thousands of free web theme for the site.
The themes are great at providing you with a base to start off your web design with. They set up the site with basic layout and dummy content which you can edit to make your own. When choosing a theme, we recommend going for a really clean, minimal theme which will look lovely. Minimal themes also involve less html in the backend, so are generally quicker and easier to edit.
Businesses with specific aims for their website might want to choose a theme for a specific purpose, such as a portfolio based theme which can display images in an attractive way, or with ecommerce funtionality. It all depends on the overall effect you want the site to have.