It consists of "two all-beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun."
The Big Mac is known worldwide. The Economist has used it, only half jokingly, as a reference point to determine the cost of living in different countries, as it is so widely available and the product (largely) comparable across markets.
In addition to being the name of a burger, Big Mac is also the name of a McDonaldland character. Dressed as a constable and sporting a large Big Mac as a head, he was the main source of law and order in McDonaldland, spending most of his time chasing the Hamburglar.