Terraform is used to create, manage, and manipulate infrastructure resources. Examples of resources include physical machines, VMs, network switches, containers, etc. Almost any infrastructure noun can be represented as a resource in Terraform.
Terraform is agnostic to the underlying platforms by supporting providers. A provider is responsible for understanding API interactions and exposing resources. Providers generally are an IaaS (e.g. AWS, DigitalOcean, GCE), PaaS (e.g. Heroku, CloudFoundry), or SaaS services (e.g. Atlas, DNSimple, CloudFlare).
Use the navigation to the left to read about the available providers.