Astronomy is the scientific study of the entire universe. While the word 'astronomy' comes from Greek for "science of the stars", astronomers study a wide range of subjects including atmospheres of planets, stars, gas and dust, individual galaxies, arrangement of galaxies, and how things have changed over the lifetime of the universe. Astronomers study a certain kind of object or behavior and try to understand how it works. Maybe this means looking at a class of asteroids, or one particular galaxy, or one kind of star in different galaxies.
The foundation of astronomy is physics and mathematics, so astronomers use fundamental forces like gravity and electromagnetism to understand what they observe in space, to find patterns, and to make predictions. This includes an amazing range of sizes where small things like sub-atomic particles, photons, atoms, molecules, and dust grains determine the behavior of large things like planets, stars, and galaxies.
Astronomy can be a passive science, for instance an astronomer interested in planet formation cannot set up a real solar system in the lab and study it from start to finish. Instead, the laboratory is the entire universe and the experiment is continuously in progress. Since many interesting objects are too far away to visit, astronomers are very good at learning as much as they can just by looking at the light from objects. Sometimes, however, astronomers can use laboratories to reproduce conditions found in astronomical settings. These experiments range from re-creating the ice on Europa's surface to heating material to the temperatures found in the hearts of stars.
Modern astronomy also makes use of computer modeling to test ideas: while it is impossible to create an entire galaxy in a lab, a rough model of one can be created on a computer. An astronomer can build a rough computer model of a galaxy with gas, dust, and stars and tell the computer the physics of how these components should behave. How this galaxy looks over millions or billions of years could then be compared to observations of real galaxies. If the model matches observations, then an astronomer would assume they have a good idea of how a galaxy works; if the model doesn't match observations, or doesn't match them all, that implies there is more to be discovered!Want to learn more? Good. A healthy dose of curiosity makes life interesting. I've written a webpage about black holes to get you started and list few more good ones: