As we already know, carbohydrates are pretty essential for an athlete that involves his/her body in intense activities.
Carbohydrates are basically sugars. We can split them up into 4 categories, monosaccharides, disaccharides, polysaccharides and fiber. Two sub-categories are simple and complex carbs.
Simple = one or two sugars (mono/disaccharides)
Complex = three + sugars (polysaccharides and fiber)
Once carbohydrates enter your mouth, they start digesting. The whole goal of metabolizing carbohydrates is to break them down into monosaccharides (glucose = monosaccharide). Once they get broken down into glucose, some of that glucose enters your bloodstream for immediate use and the vast majority of it gets stored in your skeletal muscles (muscles that attach to your bones - biceps, quadriceps, hamstrings etc.) for future use.
While you're active, your body breaks down glucose to produce energy (ATP). Glucose is your body's primary source of energy yet it's used only after your body's first energy source (phosphocreatine). High intensity actions can quickly drench your body's glycogen resources. That's why we always point out the effectiveness of half time high (simple) carb snacks and high carb post match meals.