Lambic beers are created through spontaneous fermentation. The wort is boiled in a kettle and then left in an open vat at room temperature. The yeast is not added but "captured" from the air. The temperature conditions mean that lambic beers cannot be brewed all year round.
A Geuze (or Gueuze) is a mixture of a not yet fully fermented (i.e. about 1 year old) and a 2-3 year old Lambic. The second fermentation then takes place in a bottle. It is also referred to as champagne beer by connoisseurs.