Metalloids are a group of elements that run from Boron to Polonium in the periodic table that bridge the metals and non metals. They are a group of elements that have a range of properties that are not quite metals and not quite non metals. There is not one agreed specified definition for metalloids.
A good example of this is silicon which has a luster like a metal being shiny and also conducts electricity, however it is brittle and will easily shatter like a non metal. Most metalloids like silicon have a shiny appearance and can conduct electricity but are brittle and chemically act like non metals.
Commonly they are used as alloys, fire retardants and as a semi conductors. Boron is a good example of a metalloid. It is used in combination with silicon to make Borosilicate glass which is very thick and resistant to shock. It is also used in high strength materials as it has a high melting and boiling point.
Brittle is used to describe a physical property where layers of atoms when moved across each other will break apart. Commonly in ionic lattices as the ions repel each other
An alloy is a metal structure that has different sized atoms in preventing the layers from sliding over each other and giving it different properties. An example is steel which is a mixture of iron and carbon.
Non metals are located mainly on the right hand side of the periodic table and comprise of covalently bonded compounds. Non metals share electrons to have a full outer shell.
Elements are atoms with the same number of protons in the nucleus
A metalloid is another term for a semi metal. An element that does not have properties of metals or non metals but of both. For example they can be brittle but can also conduct electricity.
Semi conductors is a term to describe metalloids that are able to conduct a current when electrical energy is applied due to the movement of electrons but the conductivity measurements are not as high as metals due to fewer electrons to carry a charge or a less ordered structure.
Luster is a term for a reflective surface that reflects light giving a shiny appearance.