Raster Graphics editing software is used to create and edit pixel based artwork.
A pixel is a single point in a picture. Pixel based artwork includes photos or any images that are "raster" formats such as JPEG, PNG, GIF and TIFF. Think of a pixel as a tile in a mosaic. Each is its own color and together many pixels build the image.
Photoshop is a popular commercial graphics editing software. Using Photoshop or similar programs you can: