Nonmetal
A nonmetal is an element that is usually dull, brittle, and a poor conductor; nonmetals gain or share electrons and include oxygen, carbon, and the halogens.
Definition
A nonmetal is an element that is not shiny, does not bend well, and does not carry electricity, and many nonmetals are gases. They sit on the upper right of the periodic table (plus hydrogen), lack metallic luster, conduct heat and electricity poorly, and are brittle when solid; at room temperature $11$ nonmetals are gases, one (bromine) is a liquid, and the rest are solids. Nonmetals have many valence electrons and tend to gain or share them, forming negative ions with metals or covalent bonds with other nonmetals, because their high ionization energies and high electronegativities make them oxidizing agents. Their solids are molecular or network covalent rather than metallic, with no delocalized electrons, hence the poor conductivity and brittleness, and nonmetal oxides such as $CO_2$, $SO_2$, and $NO_2$ are acidic, dissolving in water to form acids, in contrast to the basic oxides of metals.
Example
Oxygen and nitrogen in the air are nonmetals, sulfur is a yellow, crumbly nonmetal solid, and carbon in a pencil is a nonmetal too. Chlorine gains one electron to form $Cl^-$ in $NaCl$, an ionic compound, two chlorine atoms share electrons to form $Cl_2$, a covalent molecule, and carbon shares four electrons in methane, $CH_4$. Sulfur dioxide from burning coal dissolves in atmospheric water to form sulfurous acid, and further oxidation gives sulfuric acid, $SO_3 + H_2O \rightarrow H_2SO_4$; this acid-forming behavior of nonmetal oxides is the chemistry behind acid rain.
Key Insight
Nonmetals are few, but they are everywhere in living things, since carbon, hydrogen, oxygen, and nitrogen make up almost all of your body. Graphite is the odd nonmetal that conducts electricity, because its carbon sheets have free-moving electrons, so pencil "lead" can complete a circuit. The eight most electronegative elements are all nonmetals, with fluorine at $3.98$ on the Pauling scale, and electronegativity, more than any physical property, is what separates nonmetal chemistry from metal chemistry.