Linux's main feature is that it allows you set up a system customized for your needs. On distros such as Gentoo or Slackware you get a minimal system with just enough software to compile more stuff after installation. You can then choose from a selection of window managers, file managers, shells, web browsers, etc. This usually takes from a few hours to a few days, but you end up having full control over your computer.
Even the kernel (a kernel is like a bridge between the hardware and software) can be configured to run on your hardware and on it only, leaving you with a small binary that boots very fast.
Now, there are distros that do all this configuration for you, Ubuntu being the most popular (and in my opinion the best). But where's the fun in having someone else set your computer up for you?
Linux also comes with what's called a package manager, which is a program that handles installation and removal of other programs. Ubuntu's package manager is called apt, and it's both easy to use, and has thousands of packages for you to choose from.
If you're knew to Linux and don't want to spend a day or two configuring your computer, go with Ubuntu.
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Ok so the new comp should come with win 7 installed , I know I'm a noob, so I might consider installing Linux on top of that. Is it hard to set up?
No, not at all. The Ubuntu installer sets it up for you in just a few clicks.