Summary: Applications are used to store API credentials. To create an integration you must first register the applications that you want to integrate with. Blocks are modular pieces of code, used to build workflows. By combining ready-made or custom code blocks you will be able to quickly develop the integration logic you want. Workflows are integrations, made out of blocks. Workflows can be scheduled to run at given intervals, and they can be turned into templates for easy setup.
In short, workflows are made up of blocks which fetch data using apps registered in the user account. API calls on Canvas are (usually) not made to the external APIs directly; instead a simplified internal API is used for each app, which forwards the request to the external API. | 
