Trans fats, otherwise known as partially-hydrogenated vegetable oils (or shortening), are increasingly recognised as being dangerous - unless your aim is to clog up your arteries. This New York Times article (registration required) gives the low-down - please read it.
These fats are found in many processed foods, such as biscuits and potato crisps (US - potato chips), but in many other kinds of foods, too. Food manufacturers favour them because they are cheap and because their charactersitics make them easy to use in their automated processes. But they are not good for you. Unlike in the US, foods sold in the UK seem to have carried details of ingredients such as these (in the small print) for many years - it may cost a little more to buy products which use alternative and healthier oils and if you can afford these then you should certainly take the trouble to search them out.
Since I began following the Atkins diet (about 20 months ago) I have studiously avoided all foods which contain partially-hydrogenated vegetable oils, as Dr Robert Atkins strongly advised this in his books. In any case, as I now eat mainly Atkins-friendly food, my consumption of processed or pre-prepared food is negligible. Atkins own products (such as 'Advantage', 'Breakfast' and 'Endulge' bars) contain NO trans-fats and I do use those occasionally, when I don't have time to prepare food from scratch.
Of course most people won't be following the Atkins diet - it is castigated by some 'experts' as being dangerous (not a view I share of course, but that's a different subject) - but even on conventional diets there is something to be gained by avoiding foods which use this cheap fat for the convenience and higher profit manrgins of the manufacturers. Get into the habit of reading the small print on food packaging and avoid those which contain trans fats (partially-hydrogenated vegetable oils).