Components are React components used for composing the UI of Calypso.
They come with their own styles defined according to our guidelines, and manually loaded from the styles file in a component's folder: component/style.scss (as an example, here is the styles file of the Badge component). Structuring the user interface with these building blocks has several benefits — like allowing to quickly construct a view that is visually consistent with the rest of Calypso, and easier to iterate on.
Many of these components can be seen in action in our DevDocs: UI Components section.
Read documentation on components for more in-depth information.