Terraform is the infrastructure-as-code tool that became the default way to manage cloud resources across providers. HashiCorp's license change in 2023 (BUSL-1.1) split the community — the OpenTofu fork is the open-source continuation maintained by the Linux Foundation, and most new projects pick OpenTofu over Terraform for license reasons. The HCL configuration language and module system are identical between them.
The skills that matter beyond syntax are state management (remote backends, state locking, partial state import), module design (small, composable, with thoughtful inputs and outputs), provider-specific gotchas, and the increasingly important question of when Pulumi (with real programming language constructs) is the better tool than HCL.