Propositional Logic (or Sentential Logic) is concerned only with sentential connectives and logical operators (such as "and", "or", "not", "if ... then ...", "because" and "necessarily"), as opposed to Predicate Logic (see below), which also concerns itself with the internal structure of atomic propositions.
Propositional Logic, then, studies ways of joining and/or modifying entire propositions, statements or sentences to form more complex propositions, statements or sentences, as well as the logical relationships and properties that are derived from these methods of combining or altering statements. In propositional logic, the simplest statements are considered as indivisible units.
The Stoic philosophers in the late 3rd century B.C. attempted to study such statement operators as "and", "or" and "if ... then ...", and Chrysippus (c. 280-205 B.C.) advanced a kind of propositional logic, by marking out a number of different ways of forming complex premises for arguments. This system was also studied by Medieval logicians, although propositional logic did not really come to fruition until the mid-19th Century, with the advent of Symbolic Logic in the work of logicians such as Augustus DeMorgan (1806-1871), George Boole (1815-1864) and Gottlob Frege.