Most metals have a similar set of physical properties. They are shiny solids at room temperature, they have high melting points and high density, and they are good conductors of electricity. They are also good conductors of heat.
Most non-metals are quite different. They are often dull, and many are gases at room temperature because they have low boiling points. They have low density, and they are poor conductors of electricity, which means they are insulators. Many non-metals, such as oxygen and chlorine, are simple molecular substances with weak intermolecular forces between their molecules.
We say most because there are exceptions. Mercury is a metal that is a liquid at room temperature. Graphite is a non-metal that conducts electricity. Diamond is a non-metal with a very high melting point. A substance is normally classified as a metal or non-metal by considering all of its properties together.