I've seen a few posts about users who put their CD's in this file. When I looked at my Ubuntu gutsy sources.list, I found that every line had been commented out due to me not having an internet connection at the time.
Do I just un-comment these lines and save the file, or do I have to do something else?
How essential is the sources.list file to the running of the system?

How do you update your system? (if at all). If you plan to update your system using an Internet connection, you'll need to uncomment the lines and save the file (with root permissions). Though it is highly recommended to update your system with latest security updates, it may not be compulsory for you to do so.

However, later, if you do plan to update, you'll need to uncomment the lines, as I said earlier, before you actually update.

Yes you can just uncomment them and then run an update.

The sources list is quite essential for upgrading your system (security bugs, etc) and installing new software. Is used by package managers like aptitude and synaptic so they know where to find "packages" of software.

The sources list can refer to CDs or sites on the internet. They all become sources that you can draw on. If you now have a working internet connection then you don't need your CDs. Probably best to leave any reference to them commented out.

For the rest of them, remove the comment markers from lines that start with deb or deb-src. Make sure you don't uncomment any lines that are meant to be comments.

Now use your package management application (is it synaptic?) to update the system or install something new. It should be working.

OK that clears it up for me - thanks for your help. I un-commented all the lines, saved the file and checked for updates - it worked.