Mechanical seals are used in plumbing systems to prevent fluid leakage by maintaining pressure. There are different types of mechanical seals suitable for various applications. The main types discussed are pusher seals, non-pusher seals, unbalanced seals, balanced seals, and cartridge seals. Pusher seals are inexpensive and widely available but rely on a secondary seal. Non-pusher seals do not require a moving secondary seal. Cartridge seals are pre-installed but are more expensive and may not always fit existing settings. Buyers should understand the application and liquid to select the appropriate seal type and quality.