Websites can range in complexity from the most simple, single page site, which is nothing more than an online advertisement, to a website which lists a company's products, collects orders and sells goods.
While a single page site might be rather limited, for many applications a few pages of information might be all that is required. Such a website might be based on existing designs, with the owner's choice of colours and images added.
At the next level you might want your website to be able to collect people's enquiries, or to forward them to you by email, or do other small tasks which can be performed with on-line forms.
At the most sophisticated level your website can display items that you have in stock, allowing you to update this information as required, and then to allow users to order these items, and pay on line. You might also, with this kind of website want to have changing daily messages, a "latest news" column, or whatever seems appropriate.