The universe is all of space and time.
It is mainly comprised of Planets, Stars, Galaxies, and all other forms of matter and energy.
Observable Universe – A region of the universe in which can be observed from Earth, which is the time it has taken for the electromagnetic radiation from the objects to reach Earth since the beginning of the cosmological expansion. At this moment, at least 2 trillion galaxies are in the observable universe. If you consider the universe to be approximately 13.8 billion years old, the observable universe is the limit created by the speed of light itself, as further out the light simply has not reached us yet.
Spacetime – Any model that fuses the three dimensions of space (x,y,z) and the one dimension of time into a single four-dimensional continuum. It can be used to visualize relativistic effects such as why different observers perceive where and when events occur.