Running is a great form of cardio as you can get your heart rate up and keep it relatively constant there for a sustained period of time. It's also a great way to burn heaps of calories and generally tone your body as it not only involves toughening your legs up but also requires using your core, shoulders, back etc to be switched on
It improves the ability of the heart to be able pump blood around the body to deliver nutrients to muscles, as well as the ability of the lungs to oxygenate that blood for sustained periods.
Most people on here though don't really seem to care about that though, they are only interested in cardio as a means of losing fat so their muscles look more defined, or to cut weight to fit into a weight class, rather than as a part of allround fitness.
People who don't run also seem to believe that running for long periods burns muscle and so makes you weaker, and insist on relatively short, intense bursts of running or resistance training which seeks to burn fat in recovery more than as an energy source.