The periodic table arranges all of the known elements in an informative array. Elements are arranged in order of increasing atomic number. Order generally coincides with increasing atomic mass; the most massive element to be created, element 115, was recently confirmed after scientists announced they had created a handful of atoms of the elusive element, temporarily called ununpentium.
The different rows of elements are called periods. The period number of an element signifies the highest energy level an electron in that element occupies (in the unexcited state). The number of electrons in a period increases as one moves down the periodic table; therefore, as the energy level of the atom increases, the number of energy sub-levels per energy level increases.
Elements that lie in the same column on the periodic table (called a "group") have identical valance electron configurations and consequently behave in a similar fashion chemically. For instance, all the group 18 elements are inert gases. [Related: How Are the Elements Grouped?]
The periodic table contains an enormous amount of important information:
Atomic Number: The number of protons in an atom defines what element it is. For example, carbon atoms have six protons, hydrogen atoms have one, and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element.
Element Symbol: The element symbol is one or two letters chosen to represent an element ("H" for "hydrogen," etc.). These symbols are used internationally.
Atomic Weight: The standard atomic weight is the average mass of an element in atomic mass units ("amu"). Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass.
Atomic weight for elements 93-118: For naturally occurring elements, the atomic weight is calculated from averaging the weights of the natural abundances of the isotopes of that element. However, for man-made trans-uranium elements there is no "natural" abundance. The convention is to list the atomic weight of the longest-lived isotope in the periodic table. These atomic weights should be considered provisional since a new isotope with a longer half-life could be produced in the future.
(Source: Los Alamos National Laboratory)