Once you register your card you are at the Welcome Level and you are eligible for:
FREE beverage on your birthday
2 FREE hours of WiFi per day in store.
Once you reach 5 stars (Green Level) you get:
Free Beverage Customization (no charge syrups)
Free Refills on Brewed Coffee
Free Beverage with Whole Bean Purchase
Free Trial Offers
Once you reach 30 stars (Gold Level) you get:
Free Drink Every 15 Stars
Personalized Gold Card
Personalized Offers and Coupons
For more info on the program check out Starbucks FAQ here.