The hosts file is a little text file on your Mac, Linux, or Windows PC that is allows you to bypass certain domain names. This is usually found under /etc/hosts. For those not really wanting to drop into command-line to edit the file there is an app for that. Detours is an application by Jerod Santo @sant0sk1 that allows you to easily modify this without having to be root and doesn’t change your hosts file.
For developers this is a handy tool in case you want to bypass a connection from going out to the internet and reroute it to, say for example, a testing or development server.