Compounds can be represented in two main ways:
Chemical Formula and Molecular Model
The types of Chemical formula include:
Empirical formula, molecular formula, and structural formula
Types of Molecular model include:
Ball and stick model, and space-filling model.
There are so many different methods because each method specifies a specific aspect about a compound.
For example, the empirical formula gives the relative number of atoms of each element.
The molecular formul gives the actual number of atoms in a molecule
Structural formul shows how they're bonded to one another.
Ball and stick model shows the geometry of the molecule.
Space-filing model helps give an idea about the relative sizes of the atoms of a compound.
Learn more here: http://en.wikipedia.org/wiki/Chemical_formula