Wireheading is the direct stimulation of an individual's brain's reward center with electrical current. In both thought experiments and laboratory experiments, this makes the individual feel happy. In theory, wireheading with a powerful enough current would be the most pleasurable experience imaginable.
This raises difficult ethical questions for those who believe that morality is based on human happiness. A civilization of wireheads "blissing out" all day while being fed and maintained by robots would be a state of maximum happiness, but such a civilization would have no art, love, scientific discovery, or any of the other things humans find valuable. Anyone who would not value such a future for humanity, must consider a complexity of value stretching beyond simple happiness.
Wireheading is also an illustration of the complexities of creating a Friendly AI. Any AI naively programmed to increase human happiness could devote its energies to wireheading people, possibly without their consent, in preference to any other goals. Equivalent problems arise for any simple attempt to create AIs who care directly about human feelings ("love", "compassion", "excitement", etc).
There is some evidence that reward is distinct from pleasure, and that most currently hypothesized forms of wireheading just motivate a person to continue the wirehead experience, not to feel happy. However, there seems no reason to believe that a different form of wireheading which does create subjective pleasure could not be found.