Terraform Cloud (TFC) is a free to use, self-service SaaS platform that extends the capabilities of the open source Terraform CLI and adds collaboration and automation features.
Terraform Cloud enables connecting to common VCS platforms (GitHub, GitLab, Bitbucket) and triggering Terraform runs (plan and apply) from changes to configuration within the VCS. TFC manages state for the user, including keeping a history of changes. Terraform Cloud exposes an HTTP API that anyone can integrate with to build more automation around infrastructure change.
Terraform CLI is downloadable software that provisions infrastructure as code manually from a local machine. By itself, Terraform CLI does not provide functionality for collaboration, state history, etc. Terraform CLI can be connected to TFC via the "remote" backend to enable using and gaining all the benefits of TFC with the same CLI workflow.