SMILES stands for "Simplified Molecular Input Line Entry Specification", and was first introduced by Weininger in J. Chem. Inf. Comput. Sci.; 1988; 28(1); 31-36. It allows to specify the structure of chemical substances in ASCII.

SMILES in a nutshell:

• Elements are specified by their atomic Symbols, Hydrogen can be ommit
• Bonds are specified by "-", "=", "#" for single, double, and tripple bonds ("-" can be ommited)
• Branches are indicated by parentheses.
• Rings are indicated by pairs of matching digits
• aromatic atoms are written in lower case

There are also ways to describe more advanced concepts such as chirality and double bond configurations. Examples:

• Alcohol (ethanol) is written as CCO
• Cyclohexan (6 carbons in a ring) is C1CCCCC1
• but c1ccccc1 is benzene.
• 2-Methyl-Propane is CC(C)C