What is mod_rewrite ?

The Apache HTTP Server has a mod_rewrite module that you can use to rewrite the URL of the request that is coming into Apache HTTP Server

This module uses a rule-based rewriting engine (based on a regular-expression parser) to rewrite requested URLs on the fly. It supports an unlimited number of rules and an unlimited number of attached rule conditions for each rule, to provide a really flexible and powerful URL manipulation mechanism. The URL manipulations can depend on various tests, of server variables, environment variables, HTTP headers, or time stamps. Even external database look ups in various formats can be used to achieve highly granular URL matching.

This module operates on the full URLs (including the path-info part) both in per-server context (httpd.conf) and per-directory context (.htaccess) and can generate query-string parts on result. The rewritten result can lead to internal sub-processing, external request redirection or even to an internal proxy throughput.

If you want to use the functionality provided by the mod_rewrite module then you will have to first turn this module on by adding these line to the httpd.conf

RewriteEngine on

Then you can add the rewrite rules Ex. if you want to rewrite a incoming /test.js to say /new/test.js then you can add a rule like this

RewriteRule /test.js  /new/test.js

If your new to the mod_rewrite module then you turn on log for the mod_rewrite module by adding these lines to your httpd.conf

RewriteLogLevel 3 
RewriteLog "/tmp/rewrite.log" 

Once you do that mod_rewrite will generate log for every incoming request and write statements for what is incoming request url and if it is applying rewrite rule to it and why