Why does "Other" category constantly increase in size on iMac hard drive?

Question

Hello
I am new to Macs and bought my iMac a couple of years ago.  Since that time, I have not installed any programs and have all of my photos, music, and documents on an external hard drive.  Whenever I use my computer, I mainly use iTunes, Safari,and DVD player, though not all at once.  I also do not use the Mail app and currently have Time Machine turned off.  So, for the most part, I try to keep my usage minimal and maintain it properly (i.e., installing new updates, emptying the trash, not using any anti-virus programs, etc.). 
As of today, I have 485.77 GB free out of 499.25 GB on my hard drive.  It might seem silly to even bring up this topic, since space is not really something I have to worry about at this time; however, I do get concerned about the constant increase of the "Other" space.  As I have already mentioned, I do not have any other apps/programs installed aside from the ones that originally came with the computer, and all other categories do not change in size.
This is my first time submitting a question, and I have tried my best to search the help community for answers.  Although I have come across many discussions closely related to this topic and have tried to read different websites--such as http://pondini.org/OSX/DiskSpace.html--I still feel frustrated that I still don't know what's going on.    Could anyone explain why this is happening with the "Other" category?  Is there a way to prevent it?
I know little about computers and even less about Macs, so I would appreciate any help.    Also, I have copied down some information that might be helpful to know.  Please let me know if there's anything else I can provide.  Thank you!
Mac OS X
Version 10.7.5
Processor: 2.5 GHz Intel Core 15
Memory: 4 GB 1333 MHz DDR3
Storage:
Audio 4.3 MB
Movies 42.3 MB
Photos 133.7 MB
Apps 2.57 GB
Backups Zero KB
Other 10.72 GB  (this constantly increases)
PS.  If I added more memory, would it help my situation?

Answer 1

The best advice I can give a new user that is not familiar with the Mac is do nothing. The Mac will pretty much take care of itself. You don't need cleaners or virus protection.
Backup is key. As long as you have backup, if you make a mistake you can always restore from your backup. Just like a seat belt and an air bag protect you in different ways when driving, you need both Time Machine and a clone for full protection.
Try to learn the basics. You can learn a lot from reading about other user's issues here on Apple Discussions.
If in doubt, ask first. We're here to help you. Some actions are hard to undo.
As Csound1 mentioned don't delete anything from /var, /system or /library. BTW, var is for invisible files. They are invisible for a reason. System and Library contain the files that run your computer. No need to muck around in here to save a few kb of space.
Macintosh HD Root folders
  Applications
  Library
  System
  Users <---your user's folder is located here. It's OK to delete your files.