Bootstrap is a CSS framework.
https://en.wikipedia.org/wiki/Bootstrap_(front-end_framework)
https://en.wikipedia.org/wiki/CSS_framework
Let's say a CSS framework (aka front-end framework) is a kind of prefabricated construction kit or a workshop (tools, components and complete modules) for creating templates. It provide pre-built layouts, typographies, color schemes, components, and often JavaScript-based interactions.
Templates (aka themes or skins) are often used, as with DokuWiki, in front-end development as interchangeable user interfaces.
- Michael Sy.