An EIS or EIR contains an overview of the project, in-depth studies of potential impacts, measures to reduce or avoid those impacts, maps and technical details of the project area and an analysis of alternatives to the project. A key element of these documents is a formal process for concerned citizens to review and provide input on the project at hand.
Environmental Impact Statements (EISs) are required whenever a Federal agency proposes a major federal action that significantly affects the quality of the human environment as defined by the National Environmental Policy Act (NEPA).
Environmental Impact Reports (EIRs) are produced by public agencies at the state, city or county level. In California an EIR is required if a project has a significant environmental impact as defined by the California Environmental Quality Act (CEQA)