Apart from the spelling? Nothing. They're the same thing.
Shiraz is the Australian name for Syrah.
Australian Shiraz is quite different in style to that of its Rhone heartland. Ripe fruit, high alcohol, chocolate, black fruits and lashings of oak are typical of South Australian Shiraz; the wines of the the northern Rhone are more perfumed with aromas of white pepper and violets; lighter body and lower alcohol.
Today, the name Shiraz is fast becoming synonymous with the up-front, full-on Australian style while Syrah is perceived by some as a wine with more elegance and restraint. New World countries includng Chile and South Africa label their wines both Syrah and Shiraz although there is a trend for more and more producers to use Syrah.