All about Linux wrote an article about partitioning hard drives:
A few days back, when my non-techie friend came to visit me at home, he was amazed to see me booting into multiple OSes (4 to be exact) on my machine. He then wanted to know how I accomplished this feat. I told him about creating partitions and how these partitions play a vital role in installing multiple operating systems on ones machine. But this conversation with my friend set me thinking; why is there so much fuss on creating partitions? I think the primary reason that people face this issue of re-partitioning is because they do not think ahead about their future needs..