Using keepalived is a great way to add high availability to an application or even a hosted website. keepalived allows you to configure a floating IP (also known as a Virtual IP or VIP) for a pool of servers, with this special IP being applied to a single server at a time. Each installation of keepalived in the same group will be able to detect when another server isn't available, and claim ownership of the floating IP whenever the master server isn't responding. This allows you to run a service on multiple servers, with a server taking over in the event another becomes unavailable.
keepalived is by no means specific to Apache. You can use it with many different applications and services, NGINX being another example. keepalived also allows you to create a load balanced environment as well.
Let's talk a little bit about how keepalived...