Trees in a forest, fish in a river, horseflies on a farm, lemurs in the jungle, reeds in a pond, worms in the soil — all of these plants and animals are made of the building blocks we call cells. Most living organisms are made of a vast numbers of cells working in concert with one another. Other forms of life, however, are made of only a single cell, such as bacteria and protozoa. Cells, whether living on their own or as part of a multicellular organism, are usually too small to be seen without a light microscope.
Cells share many common features, yet they can look wildly different. In fact, cells have adapted over billions of years to a wide array of environments and functional roles. Nerve cells, for example, have long, thin extensions that can reach for meters and can transmit signals rapidly. Closely fitting, brick-shaped plant cells have a rigid outer layer that helps provide the structural support that trees and other plants require. Long, tapered muscle cells have an intrinsic stretchiness that allows them to change length as in contracting and relaxing biceps.
Still, as different as these cells are, they all rely on the same basic strategies to keep the outside out, allow necessary substances in, permit other substances to leave, maintain their health, and replicate themselves. All of these things together are precisely what make a cell a cell no matter what kind!