Mechanical seals are precision devices that prevent fluid leakage between rotating shafts and stationary housings in pumps, compressors, and mixers. They function by maintaining controlled contact between a rotating face attached to the shaft and a stationary face in the equipment casing, with a thin fluid film providing lubrication. Seal selection considers process fluid properties, pressure, temperature, and shaft speed, with configurations ranging from single seals to dual arrangements with barrier fluids for hazardous or critical services.