Multicellular eukaryotic organisms have many different types of cells that perform different functions as they combine to form tissues. However, there are two main types of cells within the multicellular organism: somatic cells and gametes, or sex cells.
Somatic cells make up the majority of the body's cells and account for any regular type of cell in the body that does not perform a function in the sexual reproductive cycle. In humans, these somatic cells contain two full sets of chromosomes (making them diploid cells).
Gametes, on the other hand, are involved directly in the reproductive cycle and are most often haploid cells, meaning they only have one set of chromosomes. This allows each contributing cell to pass on half of the needed complete set of chromosomes for reproduction.
Somatic cells are a regular type of body cell that is not involved in any way in sexual reproduction. In humans, such cells are diploid and reproduce using the process of mitosis to create identical diploid copies of themselves when they split.
Other types of species may have haploid somatic cells, and in these individuals, all of the body cells have only one set of chromosomes. This can be found in any sort of species that have haplontic life cycles or follow the alternation of generations' life cycles.
Humans begin as a single cell when the sperm and the egg fuse during fertilization to form a zygote. From there, the zygote will undergo mitosis to create more identical cells, and eventually, these stem cells will undergo differentiation to create different types of somatic cells. Depending on the time of differentiation and the cells' exposure to different environments as they develop, the cells will begin down different life paths to create all of the functioning cells of the human body.
Humans have more than three trillion cells as an adult, with somatic cells making up the bulk of that number. The somatic cells that have differentiated can become adult neurons in the nervous system, blood cells in the cardiovascular system, liver cells in the digestive system, or any of the many other types of cells found throughout the body.
Almost all multicellular eukaryotic organisms that undergo sexual reproduction use gametes, or sex cells, to create offspring. Since two parents are necessary to create individuals for the next generation of the species, gametes are typically haploid cells. That way, each parent can contribute half of the total DNA to the offspring. When two haploid gametes fuse during fertilization, they each contribute one set of chromosomes to make a single diploid zygote.
In humans, the gametes are called the sperm (in the male) and the egg (in the female). These are formed by the process of meiosis, which can turn a diploid cell into four haploid gametes. While a human male can continue to make new gametes throughout his life starting at puberty, the human female has a limited number of gametes she can make within a relatively short amount of time.
Mutations and Evolution
Sometimes, during replication, mistakes are made, and these mutations can change the DNA in the cells of the body. However, if there is a mutation in a somatic cell, it most likely will not contribute to the evolution of the species.
Since somatic cells are in no way involved in the process of sexual reproduction, any changes in the DNA of somatic cells will not get passed down to the offspring of the mutated parent. Since the offspring will not receive the changed DNA and any new traits the parent may have will not be passed down, mutations in the DNA of somatic cells will not affect evolution.
If there happens to be a mutation in a gamete, though, that can drive evolution. Mistakes can happen during meiosis that can either change the DNA in the haploid cells or create a chromosome mutation which can add or delete portions of DNA on various chromosomes. If one of the offspring is created from a gamete that has a mutation in it, then that offspring will have different traits that may or may not be favorable for the environment.