High Availibility is a term which means
your application can survive hardware and software failure.
HA usually needed by mission critical application
that must be available 24 hours a day and 7 days a week
(aka 24×7 apps).
Of course creating HA application is expensive.
Because you usually need expensive application server
like BEA WebLogic, expensive database like Oracle and
lots of expert developer.
Thanks to Linux, Java and Open Source software
creating HA application becoming much more affordable
although you still need many expert developers