Traditionally, Linux developers have had a major obstacle to overcome to get folks to try Linux - the fact that the prospective user will have to install it. These days, most computers come pre-loaded with an operating system like Windows or Apple's OS X. All you have to do is take the computer out of the box, plug a few things in and boot it up to get started.
While nerds may enjoy the challenge of installing and setting up a new OS, the vast majority of users don't and would only do so if they absolutely had to. Linux developers have responded to this by streamlining the installation process. The fact that you can boot a functioning Linux desktop from a DVD without installing anything is a good example of how ingenious the Linux folks have been in dealing with this issue. Another is the fact that most graphic installers will offer to configure a dual boot environment with your existing system, usually Microsoft Windows. Just select the Install alongside option and it's automatic.