Upgrades: What to discard?

Question

I really don't know what I'm doing when I'm downloading and installing upgrades. What do I keep that is "old" and what do I discard - if anything? For example, I downloaded an upgrade to iTunes and installed it in "Applications" on the HD. In "Applications" I now have 4 items:
1. the green note symbol listed as modified on August 25th as an application;
2. a symbol that looks like a blank piece of paper named iTunes 6.0.5 (also modified August 25) which is an alias and when I double click on that a new window opens that has a brown and yellow box titled iTunes.mpkg and a paper symbol titled Read Before You Install iTunes;
3. a separate yellow box symbol titled iTunes.mpkg dated June 16 which is an installation package;
4. a final symbol titled iTunes6.0.5.dmg dated June 26 which is a disk image which also opens into a new window containing the familiar box labled iTunes.mpkg + another Read Before.... icon.
Do I need all of these to keep things running smoothly or should I be discarding things?
I apologize in advance for the next couple of questions. As you can see, I don't know very much at all.
What is a disk image and what does it do?
What does .mpkg stand for?

Answer 1

Hi again,
I apologize for taking so long to reply. I'm really a novice with all of this and figuring out how to find the location you directed me to and then figuring out the instructions has taken hours. /Users/carpenter/Desktop/Snapshot 2006-08-27 22-45-13.tiff/Users/carpenter/Desktop/Snapshot 2006-08-27 22-45-13.jpg
If I did everything correctly, you should have a picture of the icon that appeared on my desktop when I downloaded an upgrade to Quick Time. When I click on this icon, a window opens containing the yellow box, and when I click on that, the Quick Time Installer window opens up. In the Applications Folder I also have the yellow box icon (titled Quick Time712.pkg) that also opens to the Quick Time Installer window. The date modified on that one is June 15. Should I keep both?
I have tried to click and drag the icon from the desktop into the applications folder, but all I have achieved is to create an alias. I'm afraid to trash the one already in the application folder because in all that I have done trying to figure this out, I have ended up downloading and installing the Quick Time upgrade 3 times having somehow lost it!
I hope I have done everything correctly this time.
Thanks