It is very common for two or more atoms to combine with each other to form a molecule. For a given molecule, the number of each type of atom is fixed: all oxygen molecules are made of two oxygen atoms, all methane molecules are made of four hydrogen and one carbon atom.
There a several common ways of writing a molecule. The molecular formula is just a count of each type of atom: C₂H₆O is the molecular formula for the ethanol molecule, the active ingredient in alcoholic drinks. It consists of two carbons, six hydrogens and an oxygen atom.
The structural formula is a drawing of how the atoms are bonded to each other. The structure below is the structural formula for ethanol:

The condensed structural formula is an intermediate representation of the above two. It is a formula written in a way that suggests the bonding of the atoms: ethanol's condensed structural formula is CH₃CH₂OH. Note that this is the same way that the atoms in the above structure are connected: the first three hydrogens to the carbon, the first carbon to the second carbon, which in turn is bonded to two hydrogens and one oxygen, and finally the oxygen is bonded to one hydrogen.

Example: The structural form of the molecule ethyl methyl ether is shown below. What is the molecular formula and condensed structural formula for ethyl methyl ether?

Solution: To get the molecular formula, simply count the number of atoms in the structure. There are three carbons, one oxygen and eight hydrogens, so the molecular formula of ethyl methyl ether is C₃H₈O
For the condensed structural formula, look at the way that the molecule is arranged. There is a carbon attached to three hydrogens, then one attached to two hydrogens and an oxygen, then the oxygen is attached to another carbon that has three hydrogens. Thus, the condensed structural formula is CH₃CH₂OCH₃

Information provided by: http://learn.chem.vt.edu