Stanford University
Stanford Encyclopedia of Philosophy: Modal Logic
Explains modal logic by means of logical notation. Somewhat difficult for a non-specialist to appreciate. However, the introductory section does provide a general definition of "modal logic."
Stanford University
Temporal Logic/stanford Encyclopedia of Philosophy
Discusses the temporal logic of Arthur Prior, a type of modal logic. After technical definitions, author addresses related philosophical issues. Also summarizes applications in natural language, computer science, and artificial...