What's a Cardigan ?
A cardigan is a type of knitted garment that has an open front.
Commonly cardigans have buttons: garments that are tied are instead considered a robe. A more modern version of the garment has no buttons and hangs open by design. By contrast, a pullover (or sweater) does not open in front but must be "pulled over" the head to be worn. It may be machine- or hand-knitted. Traditionally, cardigans were made of wool but can now be made of cotton, synthetic fibers, or any combination thereof.
Plain cardigans are often worn over shirts and inside suit jackets as a less formal version of the waistcoat or vest that restrains the necktie when the jacket has been removed. Its versatility means it can be worn in casual or formal settings and in any season, but it is most popular during cool weather.
Monochromatic cardigans, in sleeved or vest form, may be viewed as a conservative fashion staple. As an item of formal clothing for any gender, it is worn over a button-down dress shirt. A less formal style is wearing a T-shirt underneath.
Source : Cardigan Wikipedia