Trans fats are an unsaturated fat but the trans form results in a straight fatty acid like a saturated fat and so is more solid at room temperature.

They were manufactured in order to make a solid fat that would have a longer shelf life than saturated fats. They do occur naturally in some meat and dairy but in very small amounts. 

The body does not digest them as well as other fats and so they can cause problems such as increased cholesterol, decreased visual acuity, increased heart disease, insulin resistance (leading to diabetes), reproductive difficulties, decreased nutrition in breast milk and cancer. 

Read labels and avoid anything that contains ‘trans fats’ or says it is hydrogenated (the process of making a trans fat).