FaaS are contained in tiny light-weight containers. These functions are short-lived code components with a life cycle usually matching the timeline of a single HTTP request/response cycle.

Note that since the containers have a shorter life cycle, they live and die out completely, usually on a per-request basis. Therefore, they are stateless, could take more time to launch (couple of milliseconds) due to the container being spun up, and could be terminated by the FaaS provider after the given maximum time. This means that you are not supposed to have always running functions; that is also against the design of serverless architecture.

Containers do not have to be Docker containers, but now, some providers have started to support Docker-container-based function deployments.