In sexually reproducing animals, life originates due to the fusion of two cells from two different animals. The fusion results in the formation of zygote that keeps on dividing cell number.
The process, in which cells divide to form new cells with different characteristics and function, is called differentiation. In animals, zygote may divide over again and again to form new cells of matured organism. As the newly formed cells divide, their structure and shapes change.
Newly formed cells may differentiate into some specialized cells. A group of cell may form the cells of heart, some may form the cells of lungs, and some may became part of brain cells. The process of differentiation produces the cells that look different and became specialized to carry out specific function. Each group of cells may carry out a different function as compared to the other group.
For instance, cells of heart are specialized to carry out pumping function to facilitate blood pumping. Similarly, cells of lungs become specialized to carry out the process of gas diffusion.