What are the categories of NEPA?

Can you explain the different categories of the National Environmental Policy Act (NEPA)? I understand it's a crucial piece of legislation for ensuring environmental protection in the United States, but I'm curious about the specific areas it covers. Are there specific types of projects or activities that NEPA addresses, and if so, what are they? How does NEPA ensure that federal agencies consider the environmental impact of their actions before proceeding with them?