A Context-Free Grammar (CFG) is a formal system used to define the structure of languages, particularly in computer science and linguistics. It consists of a set of rules or productions that describe how symbols can be combined to form valid strings. Each rule specifies how a single non-terminal symbol can be replaced with a combination of terminal and non-terminal symbols, allowing for the generation of complex structures from simple components. CFGs are essential in the design of programming languages and compilers, as they help parse and understand the syntax of code. They can represent nested structures, such as parentheses in mathematical expressions, making them powerful tools for defining the syntax of both natural and artificial languages.