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:

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