About iMac

iMac is a desktop computer.

You cannot upgrade/replace parts in iMac - it is sealed, but on the 27" you can add RAM only; on the 21.5", you cannot, so you need to order it with as much RAM, hard drive, etc. as you want.

The retina display has a better resolution. Here is some info on it:

http://www.apple.com/imac-with-retina/

babowa has covered the main points nicely.

The 5k iMac is ideally suited for video editing, or wide-screen video presentation; the image is notably brighter and more detailed. You won't be able to find a pixel. The regular iMac still has a nice display, and the video presentation will still be excellent.

The 5k iMac only comes in 27", and the 21.5" Retina probably won't be available until September (or whenever it's announced).

Frankly, if you do only basic computer tasks like web browsing, email and so on, the Retina display is probably overkill. I strongly suggest trying out both computers at the Apple Store before buying. Trying different models made a big difference in my ultimate choice.

You can customize if you buy from the online Apple store and you should buy the best processor and hard drive you think you will need. If you buy the 27", you can add RAM later (and less expensive).

You cannot predict how long an electronic device will last - I hope mine will last a long time, but, I don't expect much after the three year Applecare warranty expires. Why? Because most manufacturers make sure that their products last until after the warranty expires so they don't have to repair or replace anything. Manufacturers need to make money which means selling new products, so if it lasts 5 - 6 years, that is excellent for a computer, especially since they are outdated within a couple of months or so. Also remember that hard drives can fail any time and after 3 years, it would cost you quite a bit to have that replaced in the new iMacs which are glued shut.

If you feel comfortable doing that, great for you!