RewriteRule ^cat_([0-9]*) /?cat=$1 [N,QSA]
RewriteRule ^item_([0-9]*) /?item=$1 [N,QSA]
Here's a specific .htaccess example that will do what you're looking for:
Just put the above in a file called .htaccess in your root web folder, and you'll be good to go.
Note that you may have to enable mod-rewrite in your apache server if it's not already enabled (most shared hosting providers have it enabled)
You want to use .htaccess to rewrite your URLs internally and route them appropriately to your application.