  In this paper, we briefly review the holographic dark energy model and
introduce the idea that dark energy is a kind of thermal energy related to the
quantum entanglement of the vacuum across a cosmic future event horizon. The
holographic dark energy model comes from a theoretical attempt to apply the
holographic principle to the dark energy problem and follows the idea that the
short distance cut-off or ultraviolet (UV) cut-off is related to the long
distance cut-off or infrared (IR) cut-off. The IR cut-off relevant to dark
energy is the size of the future event horizon. This model gives a holographic
dark energy comparable to the observational data. Though this model is in good
agreement with observational data, some problems (non-locality, circular logic,
causality problem, $\it etc.$) exist due to the use of the future event horizon
as a present IR cut-off. These problems of the holographic dark energy model
are considerably resolved using action principle and equations of motion.
Finally, we discuss the relation between quantum entanglement and dark energy
which is connected to the more fundamental relation between entanglement and
gravity.
