Cells are organised throughout the body to increase their efficiency.
Cells come in many types, due to cell differentiation. Differentiation is all about adapting for a specific purpose to perform that purpose most efficiently. To improve efficiency further cells are grouped by type into tissues. A basic definition would be that a tissue is a group of cells that all perform the same functions. For example muscle tissue is made up of many muscle cells, all performing the same task. Epithelial tissue is made up of epithelial cells, which coat the surface of organs and can secrete mucus.
An organ is defined as a group of tissues working together to perform a variety of functions and one quite important one. For example, a lung is an organ because it is made up of muscle, epithelial and lung tissues. Each of these tissues will contribute towards the major functioning of the lung.
It can sometimes be hard to determine an organ from a tissue: an artery is an organ because it is made up of multiple tissues, but a capillary is not since it is only made of epithelial cells.
Finally, a group of organs is known as a system. These organs will all work together to perform a function extremely efficiently. The respiratory system is responsible for the exchange of gases from the environment to the body and vice versa, the organs including the lungs and the trachea (throat). The digestive system is responsible for the digestion and processing of food, making use of the stomach and intestines.
The component cells of the body are so specialised that they are unable to perform certain tasks for themselves and this lead to the rise of organs and systems, which support other cells in the body. This has its advantages: cells can work more efficiently and, with organisation, this has resulted in the evolution of sentient organisms. The disadvantage is that every cell depends on another for survival, so damage to just one organ can kill every cell in the body.