From eagle-rock.org
A triglyceride molecule

Fats consist of a wide group of compounds that are generally soluble in organic solvents (ether, chloroform, benzene) and generally insoluble in water. Chemically, fats are triglycerides[1], triesters[2] of glycerol[3] and any of several fatty acids[4] .

Fats may be either solid or liquid at room temperature, depending on their structure and composition. Although the words "oils", "fats", and "lipids" are all used to refer to fats, "oils" is usually used to refer to fats that are liquids at normal room temperature, while "fats" is usually used to refer to fats that are solids at normal room temperature. "Lipids" is used to refer to both liquid and solid fats.

Fat is a nutrient that is an important source of calories. One gram of fat supplies 9 calories - more than twice the amount we get from carbohydrates or protein.

Fat also is needed to carry and store essential fat-soluble vitamins, like vitamins A and D.