Carbohydrates are made up of carbon, hydrogen and oxygen molecules. Carbs are found in all plant foods ( vegetables, fruit, seeds, nuts etc  ) and dairy products, whereas the foods without carbs are meat, poultry, fish, eggs. The consumption of carbs vary depending on individual; while an extremely active underweight male might need 80 grams of carbs at a meal, an overweight female may need only 15 grams of carbs.
Note: The seven major classes of nutrients are as follows: carbohydrates, protein, fats, vitamins, fiber, minerals, and water.
Carbs are the main source of fuel for the brain, kidney and red blood cells and in absence of carbs the heart, lungs and muscles start burning fat to use as fuel (also referred to as keto diet). After we run out of fat the body starts using protein to produce the glucose it needs.
Simple Carbs: Simple carbohydrates get broken down quickly to be used as energy by the body which are generally found in foods such as fruits, milk, and dairy products. Simple carbs are also found in processed and refined sugars such as table sugar, candy,  syrups, and soft drinks.
Complex Carbs: Complex carbohydrates are the sugar molecules in our body that are strung together in long, complex chains which are found in foods such as beans, peas,  whole grains, and vegetables. Complex carbohydrate foods have vitamins, minerals, and fibers which play a great role in maintaining a healthy body and mind.
Simple as well as the complex carbohydrates are turned to glucose (blood sugar) and used as energy in the body.  The formed glucose is used in the cells of the brain and body and the unused glucose is stored in muscles and liver as glycogen for later use. Complex carbohydrates are  starches and naturally occurring sugars, rather than processed or refined sugars and should be the main source of carbs, while, simple carbs do not have the vitamins, minerals, and fibers found in complex carbohydrates.
As soon as we start eating sugars and starches, our saliva (with the help of amylase – an enzyme) breaks them so by the time they reach to gut, they are ready to be absorbed as glucose into the bloodstream.
How much sugar? 

As per WHO, the free sugars should be less than 10% of our total energy intake. In certain countries the recommendation goes down to 5% of total energy intake which means 4-5 teaspoons each day if you are taking 1600-1800 calories each day.