If you are an advanced user than you must have once used the host file to configure DNS of your system. Host file is the first file that is referred by your DNS client in order to resolve a DNS domain name into IP addresses. All the entries in the host file are loaded first into the memory before resolving any dns domain name.
One major disadvantage of using modified hostfile is that it needs to be reloaded into Operating system before its entries take any effect. For that you will need to switch off your web browser first.
DNS Flusher is a firefox add-on that can be used to flush and reload host file in memory even when your web browser is open.