Vector vs Raster graphics for large format print
Designers and reprographic operators often prepare files in very different ways. Sometimes when artwork is created it is done so with a particular use in mind. For example a logo maybe created for a website but later on the client wants to use that logo for his printed brochure and posters printing. If the logo was not created in the right format initially this could cause problems with the quality of the printed graphic. The preferred format for an design that will be enlarged at a later date such as logo's or illustrations is vector artwork.
What is the difference between Vector and Raster images?
The difference between vector and raster images is that vector graphics are composed line art, or paths and raster graphics are composed of little square dots known as pixels. A vector graphic usually has an EPS file extension (Encapsulated Postscript) or and AI file extension (Adobe Illustrator) but there are other vector file format too. Fonts are composed of vector data for example. Vector line art can be straight, curved and even contain gradients. A raster image on the other hand, such as a JPEG, TIFF of GIF, is made up of an array of pixels in various colors, which when put together and viewed from a distance merge to look continuous and form the images we see every day on our computer screens and in printed magazines.
As you can see in the example above the graphic and text to the right was vector data and looks much crisper with clean edges. The graphic and text to the left was raster data and when enlarged you begin to see the pixels and blurring of the edges.
The data file for a vector graphic contains the points where the paths begin and finish, the angles that form the paths curve, and the colors that either fill or border the shapes and paths. Because vector images are not made of square dots or pixels, the images can be scaled infinitely without losing any of the quality. Raster graphics, on the other hand, become "pixelated" since each pixel increases in size to fill more pixels as the graphic is stretched bigger. This is why logos and other designs are better created in the vector format -- the quality will look the same on a business card or website as it will on a huge A0 poster.
Further demonstration on a Vector Raster comparison.
I found this great video on youtube showing the caparison of Raster and vector images and what happens when you zoom in or enlarge either of the graphics.
What to do if you want a Vector graphic but only have a Raster image or vice-verso?
Well this is easy to do one way and more difficult to do the other, but not impossible.
Convert Vector to Raster
If you have a vector image and want to convert this to a Raster image then this is easy. You can usually save or export form whichever vector program you are using (such as Adobe Illustrator) as a tiff or jpeg. Alternatively you may be able to open or import the graphic into your raster image program (such as Adobe Photoshop).
Convert Raster to Vector
Now this is much more difficult and unless you are experienced at using graphic arts software such as Illustrator you will probably find this too complicated. It can be done using automatic tools such as auto trace in Illustrator but the results are usually "okay" at best. There are other automates services online that claim to do a good job, but in my opinion the only real solution is to manually redraw the graphics in a vector program. There are many tutorials on youtube or graphic design websites or alternatively you could enlist the help of a graphic designer. A quick google search brought up this Raster to Vector drawing service.