文件夹等同于目录，对不对，为什么？

对

对。好像早期的叫目录。
补充一下，严格的讲，两个不一样。目录英文是directory：A listing of the files contained in a storage device, such as a magnetic disk.
文件夹的英文是folder：A flexible cover folded in the center and used as a holder for loose paper