Curl is a command line program for retrieving information via the http, https, and ftp protocols. It can be very useful in scripts and for testing server setups.
Testing virtual host setups prior to changing over domain names is one example. You can change the Host request header while requesting information from a particular ip address to check what will be returned for a particular host. For example, to check what is returned for testHost.org at 127.0.0.1, you would execute curl -H 'Host: testHost.org' http://127.0.0.1