BRONZE tickets correspond to a general entrance. They give you access to the BRONZE area (the pedestrian area around the circuit - where you can walk around).
BRONZE tickets do not allow you to have a seat allocated in a grandstand.
SILVER tickets and GOLD tickets give you access to the BRONZE area and to an allocated seat in a grandstand. SILVER tickets give you access to a SILVER grandstand which you must choose when you buy your ticket, and GOLD tickets give you access to a GOLD grandstand which you must also choose when purchasing.