Caravan is a Web application development platform. It has its own inbuilt
Application Server (HTML embedded scripting language)
Database
Web Server
Email
FTP
And much more....
In short we call Caravan as a "web operating system", since it has inbuilt all the "components" needed to develop and deploy a web application. No external database or web server
You can use open source third party tools (normally command line files to which parameters are passed and output results) within the application e.g image management tools, zipping tools, bar code generation or reading tools, pdf convertor etc.