Some understanding of partitioning and file systems is necessary if Linux is to be added to a computer. This understanding is likely to be in the experimenters who are attracted to rather than repelled from doing more with a PC than having just Microsoft Windows. Experimenting is necessary if one wants to have an IT career. One can learn about partitions on Wikipedia. I am noting some things here, also.
Each partition on a hard drive acts as if it is it's own hard drive, so a 200GB drive can be redone to be like four 50GB drives. You can format a data partition without affecting the other partitions on the hard drive.
Even with ordinary Windows, multiple partitions are sometimes on the drive. There may be a backup partition that can be used to restore Windows in case of a crash. This backup partition is often D:. The swap file (for Windows or Linux) is sort of a partition. But it is normal for a computer sold with Windows to have one hard drive and one partition, C:. This is a significant limitation and is a lack of flexibility for a user who might wish to have a data partition, a backup partition, or alternate operating systems.
If you install Linux onto a hard drive, as opposed to doing Linux from a "live CD" or live USB memory, Linux needs at least one partition plus a swap partition. (Linux, as opposed to Microsoft Windows, can be installed into any partition, even on a logical drive.) Early in a Linux installation, a screen appears asking if you want to install to hard drive or run Linux as a trial without changing the Windows partition. The choice for installing to HDD is followed by a repartitioning process whereby the Linux installer does something like defragmenting, opening up the requested gigabytes for new partition(s). I have always had success with this, but some people have had Windows disabled and have lost the data and programs on the Windows side. This is such a threat that the average parent needs to think hard before letting a son or daughter install Linux. Lingering users of Windows XP should know that retailers do not have XP for sale anymore; some computer shops can reinstall XP for a fee but lost data may stay lost. XP users should also know that their older computer probably does not have the memory to support Windows 7 or 8, so if XP is important on the older computer, you are quite boxed in about recovering from a repartitioning failure.
On the other hand, almost any old PC, even with 300MB of memory, can make a fine Linux box. This is a way to put an old PC to good use. A great learning setup is to put two old PCs, loaded with Linux, onto a $20 networking switch and use them to do some networking.
The basic Linux partition requirement is a swap partition and an ext2, ext3, or ext4 partition for Linux. Many books advocate having an added, two-gigabyte partition for /boot and maybe for some of the other Linux directories. It is a good idea to leave free or unallocated maybe 20% of the hard drive. Some towers have a place to put a second hard drive, for the experimenter with some cash. But be careful to buy the proper type, IDE (PATA) or SATA.
A property of a partition is that it has one file system. For a Windows partition, it is likely to be NTFS. For Linux, it is likely to be ext4. The one OS has a limited or nonexistent ability to see into the file system of another OS. A good common denominator is a partition that is fat16 or fat32 or vfat, so an additional fat32 partition is a way to pass files between OSs. A USB drive is often a fat file system, by the way. Windows 8 servers will have ReFS, and this new file system may show up on Windows 8 clients as Windows 8 marketing progresses.
Anyone doing repartitioning needs to know about the limit of four primary partitions per drive, or three if you want an extended partition with many logical drives. Windows must be allowed to have the first primary partition.
Programs such as Internet browsers and "office" software are compiled for one operating system. You can buy Microsoft Word for Windows, Mac, or Linux, but each program is only usable for the OS that it was compiled for. The great thing about Linux is that hundreds of Linux programs can be downloaded for free. Why Linux programmers put so much work into programs for which there is no way to make money is a puzzle.