very new to linux/ubuntu How to get out of terminal?

Hi

I am a very very new to linux. There are lots of things that seem so easy once you know the answer, but which are terribly difficult if you just simply don't know and can't find the answer.

Whilst researching how to download, install and use ubuntu I worked out i can use the disk as a live cd to try it out, how to burn the disk, where to get the disk burner software and the ubuntu download etc. I also found info that advised checking the integrity of the disk once it was burnt using md5sum. I even managed to look up and find the correct hash to compare it to. I thought I was armed with all the info I needed to try it out.

I successfully burnt a disk and then ran it as a live cd but when I tried to follow the commands to use the ms5sum (various commands on various pages of documentation) I had many difficulties. First it took me ages to work out how to open a terminal, but managed, then I tried lots of commands (copied from the documentation pages I had read), nothing seemed to work so I will assume I was doing it wrong somehow (a problem with my lack of knowledge rather than bugs or ubuntu not functioning properly although for someone as new as me it is very confusing) and in trying various things I ended up with several terminal windows open at once.

Eventually I found info that told me all I needed to do was use the opening menu on ubuntu to check the cd had burt correctly and I didn't even need to be in a terminal typing various commands. Great that ubunt 9.04 has this cd checking thing built in and SO EASY to use - not great that I had spent a long long time finding this out.

The outcome of this was that I had several terminals open, didn't understand anything that was in any of them and I didn't know how to close the terminals so that I could restart the system to get back to that opening menu and check the cd. I read and read and read and in the end decided to just shut the windows. This brought up a message that if I shut the terminal I would kill something - hmmm - sounded scary to me but with no other options available I just did it.

So finally to my questions:
1. How does one safely close a terminal window?
I have read that with ubuntu I am unlikely to need to delve into commands straight away because lots can be done within the user interface, but equally I have also read lots of documentation and solutions to problems involve using the terminal to type commands therefore this is something I will need to gradually learn.
I have already read 'using the terminal'
https://help.ubuntu.com/community/UsingTheTerminal
and didn't find the answer. So either it isn't there or I have info-overload-blindness.

2. Having shut several terminal windows am I likely to have caused any damage to anything?
I ran the cd check thingy after finally getting the live cd to restart and it said all was well.

3. Does anyone know of a link or piece of documentation or answer in a forum which bridges the gap between the first stage of getting ubuntu (tons of info of how to get it, what it is, etc all good) and the ability to understand all the answer pages and solutions that I am reading? I feel like I've missed something. I know I can learn, I know I can follow instructions, but in these early early days it seems that everyone else was taught something on a day I bunked off and now I am struggling to catch up.

Thank you to everyone who offers advice.