Cells are the basic structures of all true life forms. If it is alive, it has cells. The first life forms were single cells. They evolved billions of years ago and existed for billions of years before multicellular life evolved. All multicellular life forms have evolved in the last half a billion years. Before that, there were only single cells living in aquatic and marine environments.
The simplest cells are the bacteria. They have genetic material, DNA and RNA, within their cell membranes, but not enclosed in a nucleus. The earliest bacteria were heterotrophs, living on organic molecules around them, engulfing them, absorbing them and then excreting the waste products through their semi-permeable cell membranes. At some point there was perhaps a food shortage and some bacteria evolved to become photosynthetic. We call their descendants the blue-green algae and they were the first autotrophs.
Single celled bacteria are chemical factories capable of producing, from basic materials, all the fats, proteins, carbohydrates and ribonucleic acids that the cells need to function, grow and reproduce. That is why they are still so numerous and successful today. However at some point, some cells organised more complex organelles in which to carry out the various cell functions. The most important of these organelles is the nucleus. In the nucleus, all the DNA is stored and the instructions for cell functions are stored within them. The cells of all other living organisms above the bacteria, both single and multicelled, plant and animal, have nuclei. From protozoans to humans, our cells contain nuclei.
So what are the differences between the protozoans, with only one cell, and the cells of all the other plants and animals of the world? The simple answer is that single cells are self sufficient and unspecialised, whereas the specialised cells in a multicellular body cannot exist alone, but are dependent on the cells around them.
Think first of an amoeba, one of the simplest of protozoans. It has semi-permeable cell membrane through which nutrients can enter and waste products leave. It lives in the water where it can extract oxygen and give off carbon dioxide. It has a nucleus that is the control centre for all cellular activities and organelles within the cytoplasm that perform all those activities. It is sufficient unto itself. It is even a hunter, sensing prey in the water and surrounding it by moving its cytoplasm around the object and then digesting it and absorbing the contents. The amoeba is microscopic but it can do everything it needs to survive, with no outside help.
Now consider a muscle cell in your own body. It is far removed from food and oxygen. It is specialised for the job of contracting and expanding to move the muscle tissue of which it is a part. It cannot survive on its own. It needs oxygen and nutrients to be delivered to it via capillaries and the circulatory system and it needs the blood also to remove all its waste products. It can divide of course but is not part of the reproductive system that creates a whole new organism. It is a specialised part of a much larger body and cannot survive on its own.
Both the amoeba and the muscle cell share many characteristics: a cell membrane, a nucleus, cytoplasm and organelles. But they are also quite different, because one is a specialised part of a much larger body while the other is a complete organism on its own. This is the major difference between free living cells and the cells of multicelled organisms.
For more information: http://www.tutorvista.com/biology/unicellular-and-multicellular-organisms