The most important distinction is what they cover:
Buildings insurance covers the building itself. As a rule, this includes the building fabric, such as walls, the roof, permanently installed facilities (e.g. sanitary facilities) and, depending on the construction method, floor coverings. What exactly counts towards the building is set out in the specification standards. In cantons with cantonal insurance institutions, you can find these on their websites. In the GUSTAVO cantons (where damage caused by natural hazards and fire is also insured privately), the insurance company provides these standards.
Household contents insurance protects the movable property in your household. This includes furniture, electronic devices, clothing, jewelry and other personal items.
Buildings insurance protects the house, while household contents insurance covers the contents inside the house.
Only with both insurances together is your property fully protected.