Sellotape and Scotch Tape are brand names of pressure-sensitive adhesive tape. Both brand names are now used generically to refer to sticky tape. Sellotape and Scotch are thus genericised trademarks.
Sticky tape consists of a backing material like cellophane or paper coated with pressure sensitive adhesive. Sellotape and Scotch Tape stick to surfaces as pressure is applied and do not require water (as gummed tapes) or heat. The most common office sticky tape is clear with adhesive on one side, but duct, masking, heavy-duty and double-sided varieties are also widely available.