Relays are electromagnetic components that utilize low-power coils to command high-power contacts while providing electrical isolation. They have moving coils or armatures, springs, contact sets, and are categorized by their functions as power, timing, or SSR relays. Key specs include the voltage of the coil, the rated value of the contacts, and insulation resistance, which should not less than 100 mega-ohms (Ω), in accordance with UL/IEC defined standards.