Experienced qualities vary, and include fear, awareness of a "presence," chest or back pressure, an inability to breathe (hence the folkloric notion of mara-like creatures tormenting sleepers) or an inability to move (paralysis), open the eyes, speak, cry out for help or scream, or a falling/floating sensation or a feeling of tripping (as hypnic jerks or myclonic jerks are interpreted by the brain). Exploding head syndrome may also be experienced, or just an overwhelmingly loud sound, like an inexplicable buzzing or ringing sound. Other symptoms may include numb, heavy, tingling and/or electric sensations and vibrations as well as feeling something touching the body or ants crawling on the skin (formication).
The hypnagogic state is sometimes proposed as an explanation for experiences such as alien abduction, apparitions, demon oppressions, or visions.
Those who experience it and are aware (i.e. an involuntary variety of lucid dreaming) of the dream are usually - but not always - extremely terrified by the experience and are overcome with an exaggerated sense of dread and helplessness, and sometimes, the sense that if the dreamer cannot awaken soon enough to end the experience they might die of suffocation. Note that this is not the same as sleep apnea.
Some folks might say that you had a type of OBE, but its really just a sleep induced hallucination from your brain still being in REM sleep. I hope that helps some.

