The dash-core-components library is bundled with Dash and contains the most popular components - such as buttons, dropdowns, sliders, tabs etc.
The dash-daq is a separate library with additional components. There is some over-lap, but most of the components are unique. If you want to add something like a knob or a thermometer to your app, this would be a good library to add.
This library is maintained by Plotly, but it’s an example of one of the dozens of other component libraries that you can use in your app. Here are two of the most popular libraries maintained by the community:
Personally, I think the dash-daq components don’t look great in the Dash documentation - the custom CSS used in the docs (which is also a Dash app!) interferes with the CSS in the daq components, and this hasn’t been fixed in the dash docs yet. Be sure to run the examples locally to see what they will look like in your app. Here are a few examples where dash-daq components are used in some sample Dash apps: