RTDs (Resistance Temperature Detectors) are categorized by their construction (wire-wound, thin-film, coiled) and sensing material (platinum, nickel, copper), differing in accuracy, cost, and temperature range, and are wired as 2-wire (basic), 3-wire (common, compensates lead resistance), or 4-wire (most accurate, full compensation) to suit industrial, HVAC, or laboratory needs for precise temperature sensing.