Environments are the highest-level object that a user can interact with on The Platform. An Environment represents your entire hosted application or product - runtimes, databases, caches, queues, tools, and all.
An Environment consists primarily of a set of Services. At a minimum, an Environment has:
Each environment has its own isolated network, allowing those containers to find each other - and no one else. That is to say, with the exception of Service Proxies (which have ports 80 and 443 exposed to the outside world), your containers are not accessible except to each other.
An Environment can expand to have any number of services - as many as you need. An Environment does not have any containers of its own - only those of its services.