Little Snitch works in the same vein as a firewall, but the difference is that instead of restricting connections from entering, it restricts connections and data from leaving.
With Little Snitch it's possible to decide which programs can send data through the Internet, and which cannot. Each time an application tries to send information through the Internet, Little Snitch will ask for your confirmation to authorize the transmission.
Your decisions are then stored as a rule, which you can modify, create, and eliminate through the program's interface.
Little Snitch also includes a small widget that will keep you up to date about the traffic generated by each and every one of those applications with access to the Internet.
Little Snitch will not stop showing confirmation messages for certain programs that make continuous use of the Internet, so you'll need to authorize their connection from all ports in order to use Little Snitch at the same time as those applications.
