Service IP Address
A service IP address is a technique similar to a CNAME in DNS, but without the issue of waiting for entries to propagate. Traditionally, you might have a CNAME record for something like www.tjhsst.edu, which points to the web server IP address adelie.tjhsst.edu. The problem with this technique is that if you change the web server, just changing the CNAME record will not be recognized by all clients immediately. This is a problem in setups using things such as heartbeat, which require quick recognition of changes in the event of an outage.
A service IP address is an IP address that is just for a service, such as www or remote or intranet. A server acquires one of these IPs in a sub-interface, but if the server that runs the service changes, you can release the IP on one machine, and acquire it on the other; theoretically an almost instantaneous change.
In practice, however, the ARP tables on the router can cause problems, since it caches what hardware address has what IP. In the case of heartbeat, it has it's own little program called send_arp (and possibly some other techniques) to send an ARP message out, indicating the change in IP. Otherwise, if a service IP is changed manually, the ARP cache must be cleared on the router manually.