Cows, buffaloes, elephants, monkeys, dogs and cats are all examples of multicellular organisms. Humans are the most complex of all multicellular organisms. All such organisms have billions of cells, tissues and different organ systems in the body.
All multicellular organisms may be seen with the naked eye, as they are large. Multicellular organisms have evolved from unicellular organisms. They have specialized cells for performing different functions. The cells of the heart are different from the cells of the liver and the brain.
As opposed to multicellular organisms, there are unicellular organisms, such as amoeba, paramecium, viruses of various kinds and bacteria. They do not have a well-defined organ structure or organ systems within their bodies.