When different tissues work together, they form an organ of a body. Organ consists of several tissues grouped together making a functional unit. Heart is the example of an organ and is composed muscle and connective tissues. Every organ performs some special functions. For example, lungs, leaf, root are all organs.
Several organs working together in conjustion form a system. The different systems work together for the normal function of a body. The circulatory system is composed of heart, blood vessels and blood, the nervous system is composed of various organs like brain, back bone and nerves. Digestive system, reproduction system and respiratory system are some other examples.
The organs and systems work together smoothly to form a single organism. Each organism, in turn is capable of reproducing its own kind.