The name of your machine, as it will be referred to by things that look it up in DNS and hosts(5).

Every decent TCP/IP connected machine has at least one host name (you'd be hard pressed to do anything on Linux without having one!) Some will have other aliases that point to the hostname. Hostnames must be unique on a network, except in the case of clustering or failover (which is a bit trickier than it sounds) - in which case you tend to have unique hostnames on the machines, and another hostname which points to a RoundRobin lookup for them.

The machine you are connected to now has a hostname of hoiho. You're probably accessing it through its alias www however. (hoiho doesn't need to know about that alias at all, except in the case of the web server. See NameVirtualHosting.)

The hostname is set at boot by the hostname(1) command, which reads it out of a file; usually /etc/hostname.

Compare FQDN.