A smiles string is a compact description of a connection table. It is mainly useful for organic compounds, though inorganics can be described as well. 

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The string utilizes the atomic symbols to show order. Atoms that appear next to each other are assumed to be bonded. Hydrogens are implicit in order to fill valences. In the following, the smiles string is shown as well as the atomic structure, courtesy of an applet written by Rick Muller.

"C" is the smiles string for methane,

"O" is the smiles string for water

Bond orders greater than single can be shown using the symbols "=#" for double and triple bonds, resp.

E.g., carbon dioxide can be shown by O=C=O.

Branches off the main skeleton can be shown using parentheses. For example, 2-methyl butane is shown by taking the n-butane skeleton:

CCCC <= n-butane

and adding a branch after the second carbon:

CC(C)CC <= 2-methyl butane

Cyclic compounds and aromatic compounds can be shown as well.

For Example, 

Name Smiles string Structure
methane C

More  examples of the parser output are shown here.