First, to be clear, Syrah and Shiraz are the very same grape variety. So the actual issue at hand is the pronunciation of "Shiraz" - some say sh-RAHZ while others say sh-RAZZ. The second is correct, much to the chagrin of anyone that deems those hard "A" sounds are just due to my being from the Midwest. It's okay, I've got the entire nation of Australia to back me up.
The grape variety in question has disputed origination, however is notably grown in France's Rhône Valley regions. Here the grape is known as Syrah (see-RAH) and is responsible for the famous varietals of Hermitage as well as blended with other varieties in Châteauneuf-du-Pape and surrounding Côtes-du-Rhône appellations.
In 1832 vine clippings found their way from Europe to Australia given the help of James Busby, a Scottish immigrant to the land down under. Through whatever means the grape came to be called Shiraz (sh-RAZZ) and is now perhaps the most notable, and certainly the most widely planted grape in the country.
The major difference between the wines of Syrah and Shiraz is stylistic. Both wines are full bodied and display a wide range of fruit characteristics. Berries, black pepper, and spice are common in either style. In France, Syrah seems to have more of an earthy, sometimes leathery quality. Shiraz in Australia on the other hand is often fruit-driven with jammy, ripe berry flavors that can sometimes edge out the more subtle earthy notes.