EnglishDecision tables are a tool for specifying which actions to perform given a number of conditions. They represent business logic in terms of rules for connecting actions with required conditions for those actions. While they might appear deceptively simple and trivial they can be a tremendously powerful tool for defining business logic and certain types of software requirements.
This article on Decision Table Patterns by Hillel Wayne describes the general concept as well as patterns, techniques, and best practices for using decision tables.