Production involves transforming inputs like labor, machines, and raw materials into outputs. There are two types of inputs: fixed inputs whose supply is inelastic in the short run, and variable inputs whose supply is elastic. A firm's production function describes its output as a function of inputs and can be expressed as short run or long run. The law of diminishing returns states that as one variable input is increased while others stay fixed, marginal product initially increases but eventually decreases. Returns to scale refers to output changes from proportional input changes, and can exhibit increasing, constant, or diminishing returns based on the relationship between input and output changes.