Polysaccharides are polymers of sugars, where hundreds or thousands of monosaccharides are joined together by glycosidic linkages.
A glycosidic linkage is a covalent bond that is formed between two monosaccharides by a dehydration reaction — the removal of a water molecule (H + OH).
Polysaccharides are used for:
In plant cells, excess glucose made by photosynthesis is turned into starch.
In animal cells, excess glucose eaten is stored as glycogen.
We (and animals!) can get our source of sugars by eating starch. We have enzymes that can break down plant starch and turn it into glucose for energy.
2. Structural material
Plant cells have tough cell walls that contain cellulose. Like starch, cellulose is also a polymer of glucose. The difference is that it uses a different form of glucose — beta-glucose. When linked into a chain by beta-1,4-glycosidic linkages, beta-glucose forms a straight chain that can form hydrogen bonds with a neighboring chain. This makes it really strong!
Humans don't have enzymes that can hydrolyze (break) these beta-linkages, which is why we can't digest cellulose (dietary fiber). Cows, however, have bacteria in their stomachs that can do this, so they can use the nutrients of the cellulose they eat.
Certain animals (arthropods: insects, spiders, crustaceans...) also use a structural polysaccharide, chitin, to form their exoskeleton.