What is a raster or rasterized graphic?
A rasterized graphic is an image made up of pixels (little squares). All of the squares are a set size and in a set place in the image, so If you make it larger it will be pixelated and not look good.
When would I need a rasterized graphic?
You would use a raster graphic when you need a smaller file size. You generally would not get a raster graphic only from a designer. You should get mostly vectorized graphics.
What is a vector or vectorized graphic?
A vectorized graphic is created with paths or vectors, therefore, the image can be as large or as small as you need it and still look good.
When would I need a vector graphic?
You would use vector graphics for your logo, business cards, and anything else that you might want to use again in the future. A good designer would normally give you a .png (preview file & raster), a PDF for printing, and either an illustrator (AI) or photoshop (PSD) document so that you can make changes in the future (all vectorized, with the exception of photoshop).
A quick note about photoshop: While shapes, paths & fonts are scalable in photoshop, anything done with the brush tool will not be.
Common raster formats:
Common vector formats: