What is thermal conductivity and how does it vary for different materials?

Thermal conductivity (k) is a material property that quantifies its ability to conduct heat, measured in W/m-K. Metals have high thermal conductivity (copper ~400 W/m-K, aluminum ~237 W/m-K) due to free electrons. Insulators like fiberglass (~0.04 W/m-K) and air (~0.026 W/m-K) have low values. For most solids, k decreases with temperature, while for gases it increases. This property is crucial in selecting materials for heat sinks, insulation, and heat exchangers.