Hypnosis is a trance state. The word Hypnosis derives from Hypnos, the Greek God of sleep. In trance state, our critical faculties are by-passed, usually through overload of message units and influences (which may also be internally produced) resulting in various manifestations of trance, such as automatic pilot mode, inertia, or seemingly "vacant", pleasantly or unpleasantly as the case may be, suggestible or impressionable states. As children, we were more in a trance state than not and those who were our carers, teachers and/or authority figures literally were our first hypnotists, because whatever they said, did or instructed took place during a time of hyper-suggestibility when we were forming in so many ways. Whatever books we read, films we watched, music we heard and events we witnessed also deeply impressed us, as did colours, smells, textures, places we went and so on.
Emotional trance is possibly the most common form of trance that people fall into during the day, along with mental thought trance, (for example that brought about by memory and remembering). Thought and emotion trances interface and become one, compounding the effect that either would have on its own. Emotional manifestations of trance can range from seemingly serene as described above, to excited, elated/euphoric or on the less appealing side of the scale frenzied - such as when a person is in an anger trance, hypnotized by their rage.
We can deliberately or unintentionally put ourselves into trance or tip into this from any number of other influences coming from any number of sources; environmental, emotional, pleasure-based, pain-based, trauma or shock based, entertainment or media induced (e.g. television, computers etc), to substance induced. Little do most people realize that we are going in and out of various levels of hypnosis on a day to day basis from various causative factors and elements from internal to external. For example, we are all familiar with boredom trance, that "spacing out" we do when audience to something uninteresting to us or performing something monotonous or repetitive.
In conclusion, hypnosis is something that is brought about more easily than not in a myriad of ways, including, overload of stimulus, attraction, music, sleepiness, spiritual ecstasy and anything which "entrances" or in some way overloads us.
Simply put, hypnosis is a trance state in which one is suggestible. It is a state of consciousness in which all levels of consciousness may be accessed, depending on the depth and type of trance state/how it was induced.