Vector Vs. Raster

• When you save TIFF, JPEG, GIF and other bitmapped images from scans or digital photographs, you're creating documents built out of millions of tiny squares called pixels, short for "picture elements." Instead of pixels, vector artwork consists of a set of mathematical formulas to describe the shapes and sizes of its graphic components. Vector materials scale without the softening and image distortion you expect in raster graphics. Along with text, these vector elements constitute the majority of content in a typical Adobe Illustrator document.
  
  

Illustrator Effects

• When you apply a soft-edged drop shadow to a piece of artwork in Adobe Illustrator or use Gaussian Blur to distort a shape you've drawn in the program, your vector artwork now includes or becomes a bitmap itself. That's because vector-only artwork can't include raster effects like shadows, blurs and glows. The same holds true for all the effects in the lower portion of Illustrator's Effect menu and in its Stylize submenu.
  
  

Transparency

• When you create areas of partial or total transparency, allowing portions of artwork to show through other elements placed above them or letting colors interact with one another where they intersect, your transparency effects must be rasterized to make them usable in other programs. You control the quality of the rasterization and the pixels-per-inch setting -- from low to high resolution -- at which the transformation takes place. In these cases, you always should preview rasterization's impact on your graphics rather than assume you know how it will affect your work.
  
  

Placed and Exported Artwork

• Adobe Illustrator lets you link to or embed pixel-based artwork you've created in other programs so you can include it with vector and text elements you create in Illustrator itself. You also can choose raster document formats in which to export the drawings you craft in Illustrator, transforming them into TIFF or JPEG files for other uses, including website placement. You even can save your work from Illustrator into a bitmapped file format and bring that bitmap back into an Illustrator file as a placed graphic.