There are two major graphic structures, vector and raster.
Vector format is an image that is created using a mathematical equation. This equation will include instructions for all the lines, curves and points in an image. Vector graphics can be scaled to any size without loss of quality because the image is simply redrawn at a different scale. Most fonts, line drawings and logos are created originally in vector format. The most common programs for vector drawing are Adobe Illustrator and Corel Draw.
Raster format is an image that is created using pixels or dots. The resolution will determine how many dots are used. The more dots, the more detail and higher quality. When you scale a raster image, you increase or decrease the size of those dots rather than adding more dots. When scaling to large you will begin to see those dots and a loss of quality will appear. Most photographs and fine art images are raster format. The most common programs for raster drawing are Adobe Photoshop and Photoshop Elements.