As a fan of the series, I have always been curious about what happened to Harry's grandparents. James and Lily Potter were killed by a dark wizard, Lord Voldemort, who had a killing curse on them. Fortunately, Harry survived and lived to see the end of Voldemort's reign.
J.K. Rowling hasn't said much about them in her novels, but we do know that they died within days of each other. This is likely due to a contagious disease that's similar to muggle chicken pox called Dragon Pox, which kills both witches and wizards alike.
It's possible that they got it as a way of murder, or it might have been accidental. Whatever the reason, it's a pretty bad idea to have a family member killed just because they don't like the Death Eaters or aren't good enough.
As it turns out, Harry's parents are actually the last heirs of a very wealthy and well-established wizarding family who were also known for their sense of duty. They were the ancestors of Linfred of Stinchcombe, who created "Skele-gro," the bone-growing potion that helped mend Harry's broken wrist in the second book of the series.
They also were descendants of the Peverell brothers, who invented the Deathly Hallows horcruxes that Harry inherited and used in his first year at Hogwarts School of Witchcraft and Wizardry. This is a large part of why the Potters are so wealthy and influential in the wizarding world.