Open-sourced tools

I read Mark's post about open-sourced softwares and especially LINUX. These days, I am hearing about this famous LINUX more than ever and I would really like to know why the hell is it supposed to be ''better'' ?? What are the advantages of using it? For what kind of users is it useful to install such a program? Is it faster? Is it hard to install? Are there any disadvantages of using it? Thanks for answering !!