Another thing you need to know about images on the Mac is that there are two main "families" of computer graphic applications: raster-based programs and vector-based programs. Some graphics programs, and some graphics file formats created by such programs, may not fall neatly into one family or the other, but the difference between the two is essential to understanding how a Mac represents and stores images.
Raster-based graphics, also commonly called bitmap images, break down an image into a grid of pixels. Each pixel is assigned a numerical value that represents the brightness and color value of that pixel. The numerical values are used both to display the image on the Mac's monitor and to store the image on disk.
All the objects in raster-based images are part of the same image; the images are built entirely of pixels. So, if you see a line and try to select it, you won't succeed. That line is actually not a line, but a series of pixels. However, some raster-based applications enable you to use layers; layers make it possible to stack objects one on top of another so that they appear to be part of the same image, but are actually separate images "layered" on top of one another. This is good because you can edit the objects on each layer separately.
Raster-based images are also called bitmapped images. This term relates to the fact that each specific bit of information (each dot or pixel) is "mapped" onto the image. In other words, there aren't any objects in the image, only bits of information.
Raster-based images are great for storing images that contain large amounts of detail, such as photographs, art, and 3D renderings.
The most commonly used applications for raster-based graphics under Mac OS X are Apple's iPhoto and Adobe's Photoshop. Most other applications that are designed to edit digital photos are also raster-based.
Vector-based images take a different approach to represent images electronically. Rather than representing a picture as a grid of pixels, a vector-based image represents a picture as a collection of geometric shapes and stores the image as mathematical descriptions of each shape.
Because a vector-based graphic is actually a collection of mathematical equations, it is infinitely scalable. You don't have to worry about resolution in a vector-based graphic. It depends on mathematical equations for its shape rather than containing a fixed amount of information (a specific number of pixels).
As you might expect, there is a downside to vector-based graphics, and it is a big one. Vector-based images can't handle complexity, especially complexity on the level of a photograph, very well. I'm not saying that you can't create interesting, complex artwork with vector-based images?because you can. However, creating a vector-based image that contains as much detail as a photograph would be a monumental challenge.