Galaxies is a gravitational bound entity usually containing of dark matter, gas, dust, and stars. Galaxies populate the Universe known to humans. There thought to be over 100 billion galaxies in the observable universe. We live in a galaxy called the Milky Way.
A planet is a celestial body moving in an elliptical orbit around a star. In our solar system in the Milky Way there are nine planets. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. There are many more planets not known to humans.
A star is a fixed luminous point in the night sky which is a large, remote incandescent body like the sun. There are 200 and 400 billion stars and at least 100 billion planets.
Galaxies, Planets, and Stars are part of Outer Space. Outer Space is the expanse that exists beyond the Earth and between celestial bodies. Outer is Space is very unknown because our technology is not advanced enough to explore the deep end of Space.
I think I am going to read some more books, but first I need some sleep!