Most, but not all, carbohydrates come from plants. Fruits, vegetables, grains and legumes are all of plant origin. The only animal-based carbohydrates are dairy foods. Now, to answer your question, simple carbohydrates are sugars. For example, the sugars glucose and fructose come from fruits and some vegetables, lactose comes from milk and sucrose from sugar cane or sugar beets.
Table sugar is pure sucrose.
Complex carbohydrates, which are chains of simple sugars, consist primarily of starches as well as the fiber found in all plant foods. Complex carbs are foods such as grains, beans, potatoes, corn, bread and pasta. There are several reasons. Simple carbs provide very little in the way of nutrition, and they contain many empty calories. By contrast, the calories in complex carbohydrates provide many nutritional extras.