FYI, the term Lambic refers a traditional Belgian style of beer which is fermented with wild yeast, rather than adding cultured yeast. By that definition you can have straight Lambic which doesn't have fruit in it, although that isn't common. The natural fermentation produces a fairly acidic (i.e. sour) beer, so it goes well with fruit. Alternately, you can have a fruit beer that isn't a Lambic beer.
With fruit Lambics, the beer is first fermented with the grains (wheat and barley), then fruit juice is added for a second fermentation. It's a very ancient form of beer, probably predating hops as a flavouring.
Traditional flavours include Kriek (cherry), Cassis (blackcurrant), Peche (peach) and Framboise (raspberry), although I've also tried passion fruit and banana flavoured ones (Chapeau is the brewery for the last).
Belgian beers in general can be quite potent, particularly anything with 'Dubbel' or 'Tripel' on the label, so you have to watch what you're drinking. If you're used to 4-5% alcohol lagers and you drink 8-10% beer the same way you will regret it in the morning, and probably later that night, too.