The main interface of Home Assistant is a list of the current entities and their states. For each entity in the system, a state card will be rendered. State cards will show an icon, the name of the entity, when the state has last changed and the current state or a control to interact with it.
The different card types can be found here.
Sensors, when not grouped, are shown as so-called badges on top of the state cards.
The different badges are located in the file
Adding a custom card type can be done with a few simple steps. For this example we will add a new state card for the domain