Covalent Bond

Sulfur is one of the elements that has expanded octet. After the second row in the periodic table, elements may have the d sub orbit electrons. Expanded octet is an exception to normal octet rule. ie. sulfur can have more than 8 electrons surrounding it. For example, SF6 (Sulfur hexafluoride) has 12 electrons with 6 bonds.

For S04, it has 12 electrons with 6 bonds (2 double bonds and 2 single bonds with 4 oxygen atoms). For the shape and diagrams check out the websites below.

Note that all bonds are delocalized
in the sense that all bonds are shared among all oxygen atoms and thus the -2 charges are also shared among all oxygen atoms.

For more information about expanded octet:
http://en.wikipedia.org/wiki/Expanded_octet

For the shape and diagram of sulfuric acid:
http://en.wikipedia.org/wiki/Sulfuric_acid
http://www.chemguide.co.uk/atoms/bonding/shapesdouble.html