When talking about computer graphics, you are commonly dealing with raster images, also called bitmaps which are made up of a grid of colored squares. You may be working with another type of image, a vector graphic. Vector graphics are a series of points, lines, and curves that can be manipulated to make any shape. Because this image is made with mathematical information and not pixels, you could theoretically scale the artwork from the size of a business card all the way up to a billboard without degrading the quality of your image.
Typical applications of vector graphics are logos and digital illustrations, but the most common use is something you probably use every day without realizing it. Most text is vector based. This is how you can change the font and size of your text without hurting the quality.
If you are wondering if your graphics are raster or vector, the easiest way to check is to zoom in. If they remain smooth, you are most likely looking at vectors. If they start to look pixelated, you are dealing with a raster image.
A graphic can only remain vector in a digital environment, so once it is printed it is no longer vector. The printer has to convert vectors to a raster image so it can print. The advantage of providing a vector image is the printer can rasterize that image at the resolution the press is operating. You’ve likely heard that images should be 300 pixels per inch to produce good print results, yet a printer often prints at 1200 DPI. This difference in print quality between a 300-PPI image and something with a higher resolution is negligible when you’re talking about photographs. The type of visual information our eyes are looking for in a photograph is different than when we look at text. Compare these samples below of live vector text, and the same page printed from a 300-PPI raster image. This is an extreme zoom, to show you the difference in the printed results.
If you are asking if this means that a vector image is always preferable, the answer is no. While it is the best solution for many things like text, our goal here is not to have you convert photographs to vector images. As you can see in this sample image, the results of doing that will drastically change the look of your photographs.
This is that same image, run through a vector trace. The important takeaway is that you do not always want vector art, so don't think you should vector trace photographs. Each image type has its own applications.
There are other instances where a raster image would be preferred over a vector. If your vector image contains so many layers and objects that you can watch the page “draw” itself when you change pages in your pdf, you are probably better off saving a raster version of that page and using that instead. This would only be typical of a very complex illustration, or a CAD rendering that has a great amount of detail.