Galaxy is the group or collection of living and shining stars, dead stars, interstellar dust cloud, gases, and black holes in the vast universe. All the objects, stars etc. are bound to one another by gravitational attraction.
A galaxy consists of many systems like our Solar system along with a number of planets. Our galaxy is also known as the milky way.
The Earth is one planet in the solar system, which is part of the galaxy. The Earth is the third closest planet to the Sun and it rotates around the Sun and is gravitationally bound to the Sun. The physical and spatial position of the Earth seems to be the only one planet where human life is possible.
Space is the three dimensional extent that contains our universe. All the stars, planets and galaxies fill the space. Space is physical and real. It has three dimensions. Occupation of space by objects is known by volume.