Extended ALGOL is a high-level, structured programming language. Extended ALGOL has provisions for communication between programs and input/output (I/O) devices, the editing of data, and the implementation of diagnostic facilities for program debugging.

The fundamental constituents of ALGOL are the language components. These are the building blocks of the language and include, among other things, letters, digits, and special characters such as the semicolon (;).

At a level of complexity higher than language components are declarations, statements, and expressions. These are the building blocks of ALGOL programs. A declaration associates identifiers with specific properties. For example, an identifier can be associated with the properties of a real number. A statement indicates an operation to be performed, such as the assignment of a numerical value to an array element or the transfer of program flow to a location in the program out of the normal sequence.

An expression describes operations that are performed on specified quantities and return a value. For example, the expression SQRT(100) returns 10.0, the square root of 100.

At the highest level are program units. A program unit is any group of ALGOL constructs that can be compiled as a whole by the ALGOL compiler. An ALGOL program is, by definition, a program unit.