A scripting language such as Ruby, Javascript, or Python relies upon an application's source code all the time. Scripting languages have no compiler or compilation phase per se; instead, they use an interpreter—a program that runs on the web server—to translate handwritten code into machine-executable code on the fly. The link between the running application and your handcrafted code is never severed, because that scripting code is translated every time it's invoked; in other words, for every web page that your application renders.

As you might have gathered from the name, the use of an interpreter rather than a compiler is the major difference between a scripting language and a compiled language.

Translating code on the web server every time it's needed is certainly more expensive performance-wise than executing precompiled code, as it requires more effort on the part of your machine's processor. The good news is that there are ways to speed up scripted languages, including techniques such as code caching—caching the output of a script for reuse rather than executing the script every time—and persistent interpreters—loading the interpreter once and keeping it running instead of having to load it for every request.