Mitosis is the process of cell division for eukaryotic cells found in multicellular organisms. All cells come from existing cells and mitosis is how most cells divide. The only eukaryotic cells which do not divide by mitosis are sex cells a.k.a. gametes a.k.a sperm and egg cells.
The following links are to sites which can help you understand how mitosis works. The sites combine images, text and animation. Look at each site to best understand how the process works.