The Donner Party was a group of American pioneers who set out for California in 1846. They became trapped in the Sierra Nevada mountains during the winter due to heavy snowfall. The party consisted of several families, including the Donner and Reed families, and faced extreme hardships, including starvation. As conditions worsened, some members resorted to cannibalism to survive. Only a fraction of the original group reached California in the spring of 1847. The tragic story of the Donner Party highlights the dangers of westward expansion and the challenges faced by pioneers during that era.