Gueuze is a demonstration of the blenders art, it is a blend or mixture of at least 2 and usually 3 or more Lambics of different ages and often even from different brewers. Many blending houses do not brew their own beer and depend on other Lambic brewers for their raw material.
The blending is done to balance the characters and flavours of the straight Lambic and give a more complex and palatable (to many people) taste.
Since the youngest Lambics are still actively fermenting when blended and bottled the fermentation carries on in the bottle giving the final product an effervescent carbonation.
Gueuze should be quite pale and straw coloured, usually slightly hazy but sometimes quite clear, it will pour with a thick white head which won't last long.
Because they are blends the finished flavour will vary but will always be quite tart and sour with a distinctively dry finish.
A good Gueuze can be cellared for years and will improve with age, much like many wines.