The one common characteristic for all elements in the group 16 is that they have six electrons in their outermost shell. The number of electrons in the individual atoms vary depending on the element and whether or not it is an ion (an ion is an electrically charged version of an atom), but not in the outer shell. The electron configuration of all group 16 elements is written as s2p4. This means that all elements in group 16 either can accept two electrons to form an ion, or they can form two covalent bonds with other elements.
With the exception of oxygen, all the elements in group 16 are solids at room temperature. Sulfur is yellow, selenium is red or black, tellurium is white or silver, and polonium is gray. Sulfur and selenium are not metals, while tellurium is a metalloid (which means its on the dividing line of the table between metals and nonmetals, having properties of both). Polonium is a radioactive metal that does not normally form in nature, but has been created successfully in labs.
Almost all physical properties of elements in group 16 increase going down vertically on the group. This includes melting and boiling points, density and radius. This is because the mass increases down the group; an atom's mass is determined by the number of protons (along with neutrons, which can vary on an atom to atom basis; the number of protons can not) in the nucleus, and each element in group 16 subsequently has more protons (oxygen has eight, sulfur has 16, selenium has 34, tellurium has 52 and polonium has 84).
Besides being the only gas in group 16, oxygen has several other important characteristics that differ it from its group. Oxygen is the most electronegative element in group 16 (indeed, it is the second most electronegative of all elements, behind fluorine), which means it easily attracts electrons, and thus is often a negatively charged ion. This attribute allows oxygen to occur in several different types of compounds, including when it bonds with hydrogen to form water and carbon to make several different types of organic compounds.