A metalloid is an element that has properties between those of a metal and a nonmetal. Typically, metalloids are lustrous and electrically conductive like a metal, but brittle. Their chemical behavior generally falls between the other two categories.
Metallic elements are on the left side of the periodic table. (The inset is a piece of the table.) Nonmetals are on the right side. The metalloids lie in a diagonal band that runs to the lower right between those two categories. There is no hard rule for exactly which elements are metalloids, but the following are almost always included: boron, silicon, germanium, arsenic, antimony, tellurium, and more doubtfully astatine. Some elements adjacent to these are often included.
Note that the concept of a metalloid is not quite the same as a semiconductor, but there is considerable overlap. Semiconductors may be elements, but some are compounds.
A semiconductor is a material that has electrical conductivity somewhere between a conductor such as most metals and an insulator such as ceramic. Semiconductors have made possible the transistor and other key components of the electronic age.
What makes semiconductors useful is that their electrical conductivity, usually low, can become much higher under certain conditions. Higher temperature increases the conductivity (the opposite behavior of metals). Light can do so as well, hence the solar cell. The diode and the transistor are designed to transmit electrical current one direction but not the other. All of these phenomena let semiconductors act as electronic switches, and that is the basis for computer logic.
The common semiconductors are silicon, germanium, and compounds of gallium and indium.