"Slacks" is very common in the US almost as "pants" while trousers is virtually unknown.
On the other hand, "slacks" in the GB would be less formal trousers = "gatki".


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Note added at 2002-06-17 22:16:17 (GMT)
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Furthermore, \"slacks\" are of MORE formal cut than \"pants\" (in the US). \"Trousers\" is \"the old people\" word but if used it means something similar to \"slacks\".
Also I think what I said about the UK usage is just my association with the adverb \"slack\". Longman dictionary says \"an old fashioned word for trousers.
To recap:
US - formal garment = \"slacks\" or \"trousers\" (rare and antiquated)/ informal garment = \"pants\"
GB - \"slacks\" (antiquated)/ \"trousers\" (regular use).

source: Native speakers, LD of CE

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Note added at 2002-06-17 22:31:49 (GMT)
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Sorry for all the confusion about the British use of the words...
For great explanation on that aspect please see:
www.bbc.co.uk/dna/h2g2/alabaster/A129647

where they write:

\"English women don\'t wear slacks - it is really an American word. UK women wear jeans or trousers or leggings (which are those really clingy \'sports\' sort of trousers) or track suit bottoms. Slacks do exist as a word for trousers in UK, but only really old ladies wear slacks. UK slacks are almost always chocolate brown, and always made of stay-press nylon. To say \"that\'s a nice pair of slacks you are wearing\" to a woman in the UK would be taken as an insult. \"

In US English, where british "trousers" isn't used to often.