When you get down to the nitty-gritty, there are essentially two ways to make computers display pictures. In Photoshop terminology, the distinction is between pixels and paths. Other terms you may hear are "raster" (rasters are rows or lines, not reggae) and "vector." We call the stuff made up of pixels "images" and the stuff made of vectors "artwork."
Images are simply collections of dots (we call them pixels or sample points) laid out in a big grid. The pixels can be different colors, and the number of pixels can vary. No matter what the picture iswhether it's a modernist painting of a giraffe or a photograph of your motherit's always described using lots of pixels. This is the only way to represent the fine detail and subtle gradations of photorealistic images.
Just about every image comes from one of three sources: capture devices (such as scanners, digital cameras, or video cameras), painting and image-editing programs (such as Photoshop), or screen-capture programs (like Snapz Pro, the operating system, and a host of others). If you create a document with any of these tools, it's an image.
Vector artwork, also known as object-oriented graphics, is both more complex and more simple than a pixel-based image. On the one hand, instead of describing a rectangle with thousands (or millions) of dots, vector graphics just say, "Draw a rectangle this big and put it here." Clearly, this is a much more efficient and space-saving method for describing some kinds of art. Vector graphics can include many different types of objects, including lines, boxes, circles, curves, polygons, and text blocks. And all those items can have a variety of attributesline weight, type formatting, fill color, graduated fills, and so on.
To use an analogy, vector graphics are like directions saying, "Go three blocks down the street, turn left at the 7-11, and go another five blocks," while pixel-based images are more like saying, "Take a step. Now take another step. And another...." At its core, Photoshop is a tool for working with pixels, but each iteration has offered more support for incorporating vector elements that retain their object-oriented characteristics, such as shapes or type, and you can also use vector elements as selections and masks on pixel-based images.
Outside Photoshop, vector graphics come from two primary sources: drawing programs (such as Adobe Illustrator), and computer-aided design (CAD) programs. You might also get vector artwork from other programs, such as a program that makes graphs.
Crossing the line
The distinction between images and artwork occasionally gets fuzzy, because vector artwork can include pixel-based images as objects in their own right. For instance, you can put a digital camera capture into an Adobe Illustrator illustration. The image acts like an object on the page, much like a rectangle or oval. You can rotate it, warp it, and scale it, but you can't go into the image and change the pixels.
A vector artwork file may include a pixel-based image as its only object. In this situation, the file is an image that you can open for editing in a painting or pixel-editing application. Photoshop's PDF (Portable Document Format) files are good examples of this. While PDF is typically a vector file format, you can create a pixel-only PDF in Photoshop.
Just to round out the confusion, Photoshop lets you include vector elements in pixel-based images, either as stand-alone objects (like text) or as clipping paths. A clipping path in an image is invisible; it acts as a cookie cutter, allowing you to produce irregularly shaped images such as the silhouetted product shots you often see in ads (see "Clipping Paths" in Chapter 12, Essential Image Techniques).