Raisins have been around for hundreds and thousands of years but in 1876 William Thompson introduced the Lady deCoverly seedless grape at the Marysville District Fair (California). These are the same grapes we use today. They have a thin skin and are seedless. Sun-dried produces the dark raisins and oven dried and cured with sulfur produce the golden raisin.
Prior to this time the raisins included the seeds and had thicker skins but still delicious. Below I've included an excerpt from The Boston Journal of chemistry and popular science review Vol. 15-17 pg 79 ©1881:
How Raisins Are Made In California. — In Mr. Blower's vineyard, Yolo County, the grapes are allowed to remain on the vine until of a golden color and translucent. They are then picked, and put on wooden trays two by three feet in size, placed between the rows, sloping to the sun. When half dried they are turned by putting a tray on top, and by inverting them both are transferred to the new tray. When the new grapes lose their ashy appearance, and after removing the green ones, the rest are put into large sweat-boxes, placing sheets of paper between every twenty-five pounds of raisins. They are left there for two weeks, when the stems are tough and the raisins soft. The packing follows, in which iron or steel packing frames are used, the raisins being assorted, weighed, inspected, and made presentable.