Your hard disk can be split up into many separate partitions, in a similar way that a large room can be partitioned by dividing walls. Each partition, while still being on the same physical device, is treated as being entirely separate to other partitions - it is as if you have multiple hard drives installed rather than just one. In Windows, each partition has its own drive letter. You might have a C, D and E drive which, while being treated as separate devices, are in fact all stored on the same hard disk.

For operating systems to be able to co-exist on the same computer, they should be installed onto separate partitions. This means that, to dual-boot between Ubuntu and Windows, you must partition your hard disk in a suitable manner.

By partitioning your disk, you are splitting the storage capacity of your hard disk into separate, differently-sized chunks. You should choose the sizes of each partition carefully to prevent running out of storage space on a partition.

Because each partition is treated as a separate device, each partition must be formatted with a filesystem. Formatting is the process of creating a filesystem on a partition. A filesystem is a method which an operating system uses to store your files - there are many different filesystems, each with its own advantages and limitations. Common filesystems used in a dual boot system include NTFS, FAT32, and ext3. NTFS, to which Linux cannot safely write data, is the default filesystem used by Windows. Ubuntu treats this filesystem as read-only. ext3 is a native Linux filesystem that can be accessed from Windows using various tools such as ext2fs. FAT32 (also know as vfat) is a filesystem to which Linux can write safely. Hence, in a dual system, a FAT32 filesystem is commonly used as a way of sharing files between Linux and Windows.

There are two different types of partition. Primary partitions can contain only one filesystem and a maximum of four of them can be used on a hard disk. Extended partitions can contain many other partitions, but only one can be used per hard disk.

When your computer initially starts up, it quickly checks itself for problems and then passes control over to a program called a boot loader. The boot loader stores a list of operating systems and partitions stored on the computer, and decides which operating system to start up. Once it makes a choice, the operating system it selected is then in control of the computer.

When setting-up a dual-boot system, the boot loader must be made aware of the changes which are being made. Ubuntu will install a boot loader for you, which allows you to choose which operating system to start each time the computer boots up. This boot loader is called GRUB.