Trans fats are the fats that went through hydrogenation, this means, liquid oils became solids, and this is used by the industry to make some food.
The purpose of this is to get a more balanced and better texture in the final result.
They increase the bad cholesterol, and this made us more propense to have coronary heart disease.
This kind of fat is more likely to be found in fast food (fried), all pastry that uses butter and some frozen food like pizza or ice cream.