A galaxy is a cluster of stars in space. The stars are quite close together, although the distances between them are still very large. Stars are heavy, their gravity keeps them together. However, there are not only stars in the galaxy, but also gas and dust.
Our Earth belongs to a star, the Sun. Our solar system is part of a galaxy. We call our own galaxy the Milky Way. You can see it in the sky at night as a bright band. When another galaxy is visible in the sky, it appears like a bright spot. So it looks like a single star, or like a cloud or nebula.
Even in ancient times, some philosophers suspected that the Milky Way might be made up of many stars. More was not known until the 20th century, thanks to astronomy. A larger galaxy that is closest to our Milky Way is called the Andromeda Galaxy. It is made up of about a trillion stars, which is one with twelve zeros. It is about two and a half million light years away from us. That's how long it takes its light to reach us. In contrast, there are only eight minutes of light between our Earth and our sun.