Diagonal line going through groups IIIA-VIA on the periodic table
The red line is the dividing line between metals and non-metals. It is rather arbitrary; as Mendeleev himself wrote: “It is...impossible to draw a strict line of demarcation between metals and nonmetals, there being many intermediate substances.” Most modern periodic tables do not feature it.

The elements with a blue background are the metalloids. Against, it's a rather arbitrary choice, but it corresponds to elements that have some properties of metals. Wikipedia describes them in the following way:


  Physically, metalloids usually have a metallic appearance but they are brittle and only fair conductors of electricity; chemically, they mostly behave as (weak) nonmetals. They can, however, form alloys with metals. Ordinarily, most of the other physical and chemical properties of metalloids are intermediate in nature