Illustrator vs. Raster-Graphics Programs
The main difference between Illustrator and a program like Photoshop is the type of graphics the two programs create and manipulate. Photoshop works mostly with raster graphics (Photoshop 6.0+ added some vector graphics), while Illustrator works with vector graphics.
Vector Graphics and Raster Graphics
Computer graphics fall into two main categories: vector graphics and raster graphics. Understanding the difference between the two helps you create, edit, and import artwork appropriately.
In Illustrator, the type of graphic image can have important effects on your work. For example, some file formats only support raster graphics, while others work only with vector graphics. Image types are particularly important when importing or exporting images to and from Illustrator... for example, linked raster graphics cannot be edited in Illustrator. Graphic formats also affect how commands and filters can be applied to images; some filters in Illustrator will only work with raster graphics.
Drawing programs such as Adobe Illustrator create vector graphics, which are composed of lines and curves defined by mathematical objects called "vectors." Vectors describe a graphic according to its geometric characteristics. For example, a bicycle tire in a vector graphic is drawn using a mathematical equation for a circle with a certain radius, set at a specific location, and filled with a specific color. You can move, resize, or change the color of the tire without losing graphic quality because the underlying equations will compensate for your actions.
A vector graphic is resolution-independentthat is, it can be scaled to any size and printed on any output device at any resolution without losing its detail or clarity. As a result, vector graphics are the best choice for type (especially small type) and bold graphics that must retain crisp lines when scaled to various sizes.
Painting and image-editing software, such as Adobe Photoshop, create raster graphics (also called "bitmap" images). Raster graphics use a grid of small squares, known as pixels, to represent graphics, rather than equations. Each pixel in a raster graphic has a specific location and color value assigned to it. For example, a bicycle tire in a raster image is made up of a collection of pixels in a defined location, creating a mosaic that gives the appearance of a tire. When working with raster graphics, you edit pixels rather than objects or shapes.
Raster graphics are the most common electronic medium for continuous-tone images such as photographs or images created in painting programs, because they can represent subtle gradations of shades and color. Raster graphics, however, are resolution-dependentthat is, they represent a fixed number of pixels. As a result, they can appear jagged and lose detail if they are scaled on-screen or if they are printed at a higher resolution than they were created for, because changing the image size simply enlarges each pixel block.
Because computer monitors represent images by displaying them on a grid, both vector and raster graphics will displayed as pixels on-screen. The difference between the two is most apparent when printing images, or when scaling them larger on-screen.