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Common Lisp is defined by a specification (like Ada and C) rather than by a single implementation (like Perl prior to version 6). There are many implementations, and the standard spells out areas in which they may validly differ.

In addition, implementations tend to come with library packages, which provide functionality not covered in the standard. Free and open source software libraries have been created to support such features in a portable way, most notably Common-Lisp.net and the Common Lisp Open Code Collection project.

Common Lisp implementations may use any mix of native code compilation, byte code compilation or interpretion. Common Lisp has been designed to support incremental compilers, file compilers and block compilers. Standard declarations to optimize compilation (such as function inlining) are proposed in the language specification. Most Common Lisp implementations compile source code to native machine code. Some implementations can create (optimized) stand-alone applications. Others compile to bytecode, which reduces speed but eases binary-code portability. There are also compilers that compile Common Lisp code to C code. The misconception that Lisp is a purely interpreted language is most likely due to the fact that Lisp environments provide an interactive prompt and that code is compiled one-by-one, in an incremental way. With Common Lisp incremental compilation is widely used.

Some Unix-based implementations (CLISP, SBCL) can be used as a scripting language; that is, invoked by the system transparently in the way that a Perl or Unix shell interpreter is.

[Text from Wikipedia]