A molecule is a group of two or more atoms which form one unit linked together by covalent bonds. Molecules can have as as many as several thousand atoms, as in a polymer (see also DNA). A molecule typically does not have electrical charge (see also polyatomic ion).
Most molecules are composed of two or more elements, but they can also be composed of multiple atoms from the same element. Single element, bimolecular molecules which are more commonly known as homonuclear diatomic molecules  include many elementary gases, such as oxygen (O2), hydrogen (H2) and fluorine (F2). Single element compounds also include man-made compounds such as buckminsterfullerenes, which are "balls" of 60 carbon atoms, as well as natural carbon compounds like graphite and diamonds, and most metals.
Molecules are typically specified by indicating what atoms they are made of and how many of each they possess. If there is more than one of an atom, its count is given by a subscript. Water has two hydrogen atoms and one oxygen atom, so it is H2O.
Often part of the molecule is listed separately in order to show some of its properties. There are more complex, but still text based, ways of "describing" the molecule's entire structure. We also use three dimensional "ball and stick" or "space filling" diagrams to show the complete physical arrangement of the atoms in the molecule.