Website builders are tools that typically allow the construction of websites without manual code editing. They fall into two categories:

online proprietary tools provided by web hosting companies. These are typically intended for users to build their private site. Some companies allow the site owner to install alternative tools (commercial or open-source) — the more complex of these may also be described as content management systems.
offline software that runs on a computer, creating pages and which can then publish these pages on any host. (These are often considered to be "website design software", rather than "website builders".)