There is really just the one Linux, but lots of distributions. Linux only refers to the kernel of the operating system, then there are some essential programs which are classed as part of the operating system. These are generally taken from
gnu, so really the operating system bit is GNU/Linux.
The part you are talking about is called a distribution. Rather than just being an operating system it is a whole collection of different applications, including a web server, a desktop environment, office suite etc. This is often referred to as "Linux", but it incorporates far more than just an operating system.
There are a number of differences between the different distributions depending upon your needs. No one distribution is better than the others, it all depends on what you want to do with your system and what support you expect.
Support and Cost
Some distributions are completely free, but then rely on community support or charge for support whereas others charge a license fee per installation. Effectively you are not really paying for the software at all, just for someone to help you out if you get stuck / find a problem.
Different Roles
There are hundreds of free programs available, and someone has to decide which software goes into the distribution and which doesn't. Different people have different needs. Some distributions are tailored towards the desktop, including a desktop environment, office suite and photo editor, others are geared towards the server environment including a web server, database and samba (windows networking) server
Some others try to do all of these, or some try and put as many software programs as possible (up to 7 CDs), or as few as possible (single floppy disk).
Different Software
More specific than having different roles there are often different applications that do the same thing. For example there are different desktop environments (KDE and Gnome), the choice of which is often personal preference. There are different programs for playing music, or burning to CD and even different web servers (although Apache is by far the most common).
Different Package Management
The way that software is installed is handled differently. The two main ways being RPM (based on a RedHat solution) and Dpkg (based on Debian). Both have their pros and cons and supporters.
When it comes down to it, it is not about which distribution is better, but about which one fits your requirements. More importantly it's about choice and the freedom to do what you want with the operating system.