Starbucks syrups are liquid sugar infused with various flavorings. If you're seeking alternatives, you're in luck: Starbucks does offer sugar-free syrup options.
The sweetness of these syrups comes from artificial sweeteners like maltodextrin and sucralose. The only option currently "officially" available is vanilla.
To make sure that you're not missing out on any special sugar-free flavors that are location-specific, you should ask the barista about the available options.