Cards are the most basic building component of a bot. They can contain anything from a simple text message or image, to other plugins which allow more advanced actions.
You can see all the available cards in the Add a card menu of each block. Clicking on the + will reveal more plugins.
Blocks are containers for one or multiple cards. When a block gets triggered, a bot user will receive all the cards which are contained in the block.
You can link blocks to each other using buttons in text cards or gallery cards — or using plugins like the Go To Block plugin. This will create a flow.
You can see all of a bot's blocks on the left of Automate section in the dashboard. The cards inside a block can be seen on the right by clicking on the block's name.