Every maOS install includes a recovery partition which works as you describe. However, if your hard drive is screwed up thoroughly enough, the recovery partitio

Every maOS install includes a recovery partition which works as you describe. However, if your hard drive is screwed up thoroughly enough, the recovery partition may not be accessible. In that case, you can still access Internet Recovery, which is located on some sort of ROM and allows you to redownload a working recovery image from Apple's servers.