The Structure of the Universe:
Solar System - Our Sun along with everything trapped within its gravitational field: the planets, the Asteroid Belt, the Oort Cloud, and lots of additional space garbage.

Planetary System - Similar to our Solar System, but orbiting a star other than our Sun.

Galaxy - A collection of stars, planetary systems, and other space garbage--including dark matter--orbiting a common center of gravity (often a black hole). Our Solar System is part of the Milky Way galaxy, which contains probably between 100 - 400 billions stars and associated planetary systems.

Galactic Cluster - Just like it sounds: a group of galaxies clustered together relatively close to each other. Our Milky Way galaxy is part of a group of around 30 galaxies called the Local Group ("local" being about 10 million light years across).

The Visible Universe - Everything that we can see or know about, it currently extends outward from Earth about 46.5 billion light years in every direction. Everything in the Visible Universe obeys the same rules of physics. It is the only universe that we are certain exists.

Other Terms:
Light Year - The distance that light travels in a year, about 9.5 trillion kilometers or 6 trillion miles. This is a measurement of distance, not of time.

Dark Matter - Matter that, inexplicably, does not interact with light. Its exact nature is currently still a bit of a mystery.

Dark Energy - A mysterious force, probably relatively weak, that causes matter to accelerate away from all other matter. It is easily overwhelmed by gravitational attraction; consequently we see its effects only when we are far away from any source of gravitational attraction.

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