Expressions are fundamental constructs in programming and database languages that represent computations or evaluations. They consist of variables, operators, and function calls, which combine to produce a value. In SQL, expressions can filter records, perform calculations, and manipulate data types, such as strings and numbers. Similarly, in programming languages like Java, expressions are used for calculations and can include literals, variables, and method invocations. Understanding expressions is crucial for constructing effective queries and writing efficient code, as they form the building blocks for more complex operations and logic within applications and databases.