What is the difference in the hard drive and the memory of a computer?

I'm planning on buying the new iMac as my first computer in a couple months. I've never understood how the two are different. Like on an iPhone it just has memory and no hard drive. Also as part of my initial question in your explanation, if I download adobe photoshop then does it go to the hard drive or the memory? What about my iTunes music downloads and pictures saved to the computer?

My Wi-Fi on my iPhone 4s isn't working

Yesterday it was greyed out, I couldn't turn Wi-Fi on or off, then I decided to try the freezer trick, I put it in the freezer for about 10 minutes then I took it out and my Wi-Fi worked the whole day, but now it's doing it again. Is it something wrong with the phone itself or will the next update fix this?

1st call goes straight to voicemail, second one connects? New iphone?

My girlfriend just bought a secondhand iphone 4s which is almost brand new. But ever since she got it, i have to call her twice to actually be able to talk to her. On first attempt, it goes straight to voicemail, and on second attempt, i can hear the dialtone.
its costing double and also an inconvenience.
is it a problem with setting, or anything else. She had a vodafone sim on that phone for a few weeks, same problem, now with TPG. Still same problem.
can anyone please help?

IPhone 5 and Early Termination Cost?

I'm T-Mobile, and I'm intent on getting an iPhone 5.
Could I technically go to an Apple store, buy an iPhone for $200, cancel the contract for $350, and pay whatever the first month's cost is? (which I assume is roughly around $100?)

That would bring me to a grand total of $650, which is still cheaper than what I find on eBay or Craigslist.

Is there something I'm missing from this picture?

My iPhone says it's low on memory?

I just got my broken iPhone replaced with a new one and I have 300 pictures on it and 5 apps and no music yet and it says it's low on memory? It's so low it won't do the update! It's a 16 GB phone and it says I've used 12.9GB in Settings>General>Usage>Storage I just don't see how that's possible!

ITunes applying wrong album artwork when music is transferred to iPhone?

I have music in my iTunes library, I want to put this on my iPhone. I check in my library to see if the correct artwork is there, it is. Then when I put the music onto my phone, It has the artwork of another album I also have. I have tried several different albums to see if this would keep happening and it does. Is there any solution to this?