In one sentence
Stars are enormous glowing spheres that create light and elements, while galaxies are vast communities of stars, gas, dust, and dark matter held together by gravity.
A simple example
Think of a star as a glowing cosmic furnace, and a galaxy as an enormous city containing billions of those furnaces, all traveling together through space.
Go Deeper
Stars form when gravity pulls great clouds of gas together until their centers become hot and dense enough for nuclear fusion. Fusion releases the light and heat that make stars shine, and over their lifetimes stars create many of the chemical elements found in planets and living things. Galaxies contain millions, billions, or even trillions of stars, along with gas, dust, planets, black holes, and large amounts of unseen dark matter. Gravity holds these vast systems together and shapes their motion across the universe.