A cell is the basic unit of living organisms that is capable of reproducing itself exactly. That is, although eukaryotic cells (such as plant cells and animal cells) contain components, which are called organelles, those components do not reproduce themselves independently of the cell of which they are a part.
"Microvilli" is the pural form; "Microvillus" is the singular form.
Microvilli are hairlike projections on the outer-surface of cells. They are typically about 5um long and serve the purpose of increasing the surface area of the cell. They are particularly apparent on the surfaces of absorptive and secretory cells. In some cases (certain areas within the organism), microvilli form dense coverings known as "brush borders" on the free surfaces of cells.
Not all cells have microvilli.
See also the functions of cell organelles.