The Universe consists of all currently known space, time, matter, and energy and all of the inherent laws and mechanisms which govern said phenomena. Different cultures and people all throughout human history have attempted to understand and define the universe, typically trying to relate what they see in the world around them to the rest of existence. The result was the emergence of various different philosophical viewpoints, which gave rise to the early religions, as well as (ironically) early versions of scientific experimentation and thought. While humanity now understands much about the universe, more new discoveries emerge daily and we continually learn that we still know very little about it.
Space and time
The universe, at its core, is composed of two main components: space, which is the physical aspects (which includes nothingness, as well); and time, which- put very simplistically- is the progression of events from cause to effect and past to future (though in actuality it is far more complicated). Space is composed of three visible dimensions- length, width, and depth- and new evidence from such fields as String Theory suggests that seven other dimensions also exist, though the exact nature of such dimensions is as-yet unknown. Time, again according to String Theory, is composed of two dimensions, one linear, upon which all known matter and energy flows forward through time, and one curved, which can come into play during time travel and/or inter-universal travel. Space and time are interwoven together at all points in the universe to form a 12-dimensional fabric known as spacetime.
Currently, our universe is the only known one, though theoretical physics and quantum mechanics predict that an infinite number of parallel universes exist. These universes exist inside a medium known as hyperspace, and are collectively referred to as the multiverse.