There are four types of expressions: literals, integers, variables and built-in or user defined function calls.
A literal is any string enclosed in double quotes; its value is itself. An integer is any integer constant; its value is itself. A variable is a named location that can be assigned different values during program execution. The value of a variable is the last value assigned to it. Variables do not have fixed type; at different times in a program, the same variable may be assigned data of completely different types. An identifier followed by comma-separated list of expressions enclosed in parentheses, is either a call to a built-in function or a call to a user-defined function.