:In an interpreted language your source code is read in command by command by a software tool, which is called the Interpreter and executed.

What ? Command-by-command ? No interpreter is using such crappy technique nowadays. Taw

 From Interpreted Language/Talk 
 
some languages (like Perl) are compiled at runtime

Could you elaborate ? Taw

Every time a perl script is run, Perl compiles it and then runs the compiled code. At least, this is the usual way of using perl, although it's not the only way. Zundark, 2001 Dec 15

You mean compile to syntactic tree or compile to machine code ? Taw

 I believe it's compiled to bytecode, much like Java. As of perl 5.6 (version?), it's possible to produce a bytecode version of a perl program to save the compilation step. I think the bytecode can be converted back to full source as well, but I'm not as sure about that. Indigo Perl (http://www.indigoperl.com) produces a perl compiler that produces a machine executable, but I think the resulting executable consists of both the actual bytecode and enough of the perl engine to run the bytecode, so they tend to be rather large. Their advantage is being able to deploy the code to machines that don't necessarily have the Perl interpreter installed.  They have versions for both Windows and Unix-ish platforms.  Wesley

 Old Interpreted Language article 

An interpreted language is a type of programming language that is not compiled into machine code, but is interpreted at run-time by the computer, and translated into machine understandable code as required.

It is often a mistake to refer to a language as either "interpreted" or "compiled", because
most languages can be implemented in either way, and some languages
(like Perl) are compiled at
runtime but behave as if they were interpreted.
Other languages, like Java, may be partially compiled into an
intermediate form which is then interpreted (although that, too, might be compiled).
Some languages, though, are specifically designed to favor interpretation.

An example of an interpreted language is JavaScript.

Is Java interpreted, or what? 

The Java programming language article says:

"The first implementations of the language used an interpreted virtual machine to achieve portability, and many implementations still do. These implementations produce programs that run more slowly than the fully-compiled programs created by the typical C++ compiler and some later Java language compilers, so the language suffered a reputation for producing slow programs. More recent implementations of the Java VM produce programs that run much faster, using multiple techniques."

The Java language is compiled, always.  It is designed as a compiled language, and is implemented as a compiled language.  It is often compiled into bytecode, which is then interpreted (with a virtual machine).  I think the Java language article is correct, but maybe it could be worded better.  LDC

Yes, and you can also compile Jave straight down to machine code if you like. The above claim that it is a mistake to differentiate between interpreted and compiled languages is IMHO incorrect; every language that has an "eval" statement which lets you construct and evaluate statements on the fly can never be cleanly compiled down to machine code; you will always have to embed an interpreter in your compiled code. AxelBoldt

Way too much mess here has been caused by lack of clear definition of "compiling".
It is used in two distinct meanings, as in:
 compiled to machine code (or assembly)
 compiled to byte code/parse tree/some other internal representation

I think it's wrong to call languages that use the second way "compiled". In such case there would be virtually no interpreted language in use - no language uses line-by-line parsing nowadays, all "compile" source to some form of internal representation.

Quote from FOLDOC:
     Compiler

     A program that converts another program
     from some {source language} (or {programming language}) to
     {machine language} (object code).  Some compilers output
     {assembly language} which is then converted to {machine
     language} by a separate {assembler}.
     ...