Mitosis is the type of cell division the purpose of which which is that two identical copies of a cell are formed. The end result is that the DNA/chromosomes replicate and one set of chromosomes, with some of the cytoplasm and its contents, goes to each new "daughter" cell.
In one-celled organisms such as bacteria, mitosis is used to produce two new, separate bacteria. In multi-cellular organisms, such as plants, fungi, and animals, mitosis is used for the growth and development of the organism. It is also used for repair when an injury occurs, or just to replace cells that have reached the end of their life span.
Mitosis happens in a set series of steps. For details on the steps (prophase, metaphase, anaphase, telophase, and cytokinesis), see the link I have attached below.
Reproductive cells of multi-cellular organisms undergo a different type of reproduction (meiosis) that yields cells with only half the required number of chromosomes, but when they join with another reproductive cell, the full number of chromosomes is restored.