Debian.
2.3 Partitioning Your Hard Drive
Before you install Debian on your computer, it is generally a good idea to plan how the contents of your hard drive will be arranged. One part of this process involves partitioning your hard drive.
2.3.1 Background
Partitioning your disk simply refers to the act of breaking up your disk into sections. Each section is then independent of the others. It’s roughly equivalent to putting up walls in a house; after that, adding furniture to one room doesn’t affect any other room.
If you already have an operating system on your system (Windows 95, Windows NT, DOS, etc.) and you want to install Debian GNU/Linux on the same disk, you will probably need to repartition the disk. In general, changing a partition that already has a filesystem on it will destroy any information in that filesystem. Therefore, you should always make backups before doing any repartitioning. Using the analogy of the house, you would probably want to move all the furniture out of the way before moving a wall or you risk destroying your furniture. Luckily, there is an alternative for some users; see
Debian Gnu/linux : Guide to Installation and Usage · The Wunder Library — complete classics, free to read, with narration.