Although Illustrator and Photoshop might seem very similar, they're actually quite different programs. Illustrator works with images called 'vector images.' Basically, it uses algorithms to determine the angles of curves, so, no matter how far you zoom in or out, you never lose image quality. In addition, the lines naturally look really smooth and crisp. A free vector program you can use as an alternative to Illustrator is Inkscape. Photoshop, on the other hand, is a raster program. Basically, the images are made up of little dots and form an image called a bitmap. Zooming in, therefore, zooms in on the dots and can make your image loose it quality. Actually, Photoshop uses the exact same kind of photo manipulation that MS Paint uses; it just does it better, and has a lot more functions. An alternative, free raster program is called The GIMP. It offers many of the same features as Photoshop, and many people choose to use it instead. But photoshop is the widely used application and most designers stick to it. Both Photoshop and Illustrator for different parts of the same design can be useful - you don't necessarily have to just stick to one program for a design. Each program has things that can make doing a certain task easier than it would be with the other.