Cells are the base components of all life, from the smallest of bacteria to the largest of trees and whales. They contain organelles which are specialized structures that serve a variety of different functions. All the major organelles are labelled in the diagram to the right.
The nucleus contains their genetic material, such as DNA and serve in many ways as the brain of the cell. Without them a cell cannot reproduce, it cannot produce new proteins and hence it cannot repair itself. All human cells, except red blood cells, have at least one nucleus, although some may have several nuclei (e.g., liver cells and skeletal muscle cells).
The mitochondria (or in singular form, mitochondrion) are a cell's "power station", where all of a cell's energy is produced.
The smooth endoplasmic reticulum is where enzymes that metabolize lipids, drugs, pesticides and carcinogenic chemicals reside (for the last three these roles are specific to the smooth ERs of the cells of the liver and kidneys). It is also here where cholesterol, lipoproteins and steroid-based hormones are synthesized.
Lysosomes are basically like the stomachs of a cell, where digestive enzymes are primed and rearing to go, they are especially prevalent in phagocytic cells like macrophages and other immune cells. They also breakdown unused/non-functioning or worn-out organelles and glycogen.
Peroxisomes are where oxidase and catalase enzymes reside, which are involved in the detoxification of certain compounds like alcohol and formaldehyde and free radicals like hydrogen peroxide. They are more abundantly expressed in the liver and kidneys.