Wikimedia
Wikipedia: Theorem
Wikipedia offers a detailed description of a theorem, or a statement that can be proven true. Includes many terms related to theorems, also.
Other
Boolean Propositional Logic
Introduction to classic, boolean propositional logic. Written in a clear way with little or no technical notation. A great place to start for those beginning to learn about logic.
Other
R. B. Jones: First Order Predicate Logic
Great introduction to basic ideas in predicate logic. Conveys fundamental ideas without using technical notation. A good place to start.
Stanford University
Stanford Encyclopedia of Philosophy: Logical Form
A terrific introduction from the Stanford Encyclopedia of Philosophy to the conventions of notation and form in logic, and indeed to logical philosophy in general. Discusses the logical semantics of propositions, description, analysis,...