Cards are embeddable, no/low code mini-apps you can create in Power Apps. They are based on Adaptive Cards, but are built with the no-code designer and Power Fx formula language used across Power Apps & other Power Platform tools.

You can get started building cards at https://make.powerapps.com/. The Microsoft documentation for Cards contains some tutorials, which you can use to get started building your first card.

At the time of writing this post, the Cards you create can be shared with your colleagues to use/interact with via Microsoft Teams. You can post it within a chat or channel. To have the card appear as shown in the screenshot below, the chat or Team you post to should have Power Apps app added to it. In the future, they will be available share via other channels such as Outlook.

Cards can be standalone, or can be connected to a data source. At this time, Dataverse is the only available data source, which is good news if you use Dataverse or Dynamics 365.
The purpose of a card is to allow colleagues to perform a simple, quick action. Unlike a regular Power App, where you might have multiple steps, screens, data sources, and advanced searching capabilities. Here are some ideas for when you could use Cards:
- Gather feedback: Provide a space for colleagues to submit responses which can be viewed within the card
- Gather votes: Provide a list of options which colleagues can make a selection from
- Display information from Dynamics 365/a Dataverse table: an account, contact, case
- Update information for a record in Dynamics 365/Dataverse within Teams