What is mitosis?

is the method by which cells make exact copies of themselves during cell division. The exact copies of the cells are needed for growth, development, and repair of tissue damage.
Mitosis is an orderly series of events whose purpose is to evenly divide the chromosomes of a parent cell between two new daughter cells. Just prior to mitosis the cell undergoes cell division, forming copies of each chromosome (called sister chromatids). This ensures that there are two copies of each chromosome prior to cell division. The events of mitosis include:
prophase: A preparation phase during which the nuclear envelope dissolves, chromsomes condense, and the spindle fibers form. (Sometimes prophase is further divided into prophase and prometaphase. )
metaphase: A brief phase during which the chromosomes align along a central line in the cell.
anaphase : The sister chromatids separate and migrate to opposite sides of the cell.
telophase : The reverse of prophase – the nucleus reforms, the chromosomes decondense and the new daughter cells start to form.
In many organisms, mitosis is followed by cytokinesis, which divides the organelles and cytoplasm of the parent cells into the daughter cells.
For a review, see the following video