cURL is one of the most powerful PHP extensions. It stands for Client URL, and allows you to communicate with other servers using a wide range of protocols. Perhaps that sounds fairly uninteresting, but give it some more thought. Other servers and other protocols? At some stage in a novice developer's career, there comes a time to break out of the local server, and cURL is the first thing you should consider.
If you are getting an undefined error it is likely that you are using a shared environment and this function is disabled see here;
For full instructions on the curl functions c here;