A galaxy is a massive collection of stars, gas, dust, and dark matter, all held together by gravity. Galaxies come in several main shapes: spiral galaxies with sweeping arms, elliptical galaxies shaped like giant ovals, and irregular galaxies with no defined shape at all.
Our Solar System sits inside the Milky Way, a spiral galaxy containing an estimated 100 to 400 billion stars. It takes light about 100,000 years to cross from one side of the Milky Way to the other, and our Sun takes roughly 230 million years to complete one orbit around the galaxy's center.
The universe contains an estimated two trillion galaxies, each one home to billions of stars and, quite possibly, countless planets. The nearest large galaxy to our own, Andromeda, is on a slow collision course with the Milky Way, though the two won't merge for billions of years.