Everything an animal is made of, built up one level at a time. Cells make tissues, tissues make organs, organs make systems.
Tap any level.
A tissue is a collection of more than one specific cell forming a structure. There are four kinds, and everything in the body is built from them.
If you are writing this up for an assignment, this is what each grade is asking for. It builds — each one is the previous one plus something extra.
It is the difference between two sentences:
Describing: the small intestine is lined with villi, and it absorbs nutrients.
Linking: the small intestine is lined with villi, and because those folds massively increase the surface area, far more nutrients can be absorbed than a smooth tube would manage.
The second one uses the word because. That is usually the whole difference.
Grades are only relevant if you are being assessed. If you are here to learn about animals, ignore this page entirely — it takes nothing away from the rest.