Monosaccharides are simple sugars that can be classified based on their number of carbons as trioses, tetroses, pentoses, or hexoses. Disaccharides consist of two monosaccharides linked together, while oligosaccharides have a few monosaccharides linked together. Polysaccharides are long chains of monosaccharide or disaccharide units that serve as energy storage molecules in plants and animals.