What are the differences between IP address 0.0.0.0 and 127.0.0.1?

Example

Let say we have a machine that has two IP addresses (192.168.1.18 and 10.1.2.1) and the apache web server runs on it. Now if we allow the apache webserver to listen on 0.0.0.0, it will be reachable at both of those IP addresses.

When a service is listening on 0.0.0.0 this means the service is listening on all the configured network interfaces.