Some more 24inch Imac ?'s

Hello again,
A few more questions before I complete my purchase

1. What exactly are dead pixels? Are they pixels on the lcd that do not work?
Are they very easy to spot or is there a program that can check for them?
2. Buzzing when reducing the brightness on the screen..fact or fiction. Its very hard to check for this at the apple store...
3. Any program I should run out of the box to make sure everything is working as it should.

I am going back to mac after using a crappy pc laptop for the past 6 years, so sorry if my questions seem extremely stupid.
Thank you every much!

1. Dead pixels are pixels that never turn on. Stuck pixels are on but in the wrong color. You might not notice them but once you do notice the pixel it sticks out to you. You can look around and I think there are some programs. I haven't heard of many problems with dead pixels on the new iMacs (My dad has none with his 24")
2. Never heard of it
3. Nothing that I can think of, just get up and go