A raster image is a way to represent photos or illustrations as digital images. If a raster image is enlarged too much, it will appear to be jagged, fuzzy or low quality. Vectors use points, lines, curves, and shapes to represent images in computer graphics. Vector graphics can be enlarged without a loss in image quality.

use it in real life: Lets say you have a logo thats a vector file and a photo image thats a raster file. Now lets say you want them to be used in a print ad. Based upon its resolution, the photo can only be enlarged so much and then it will loose its image quality and become jagged or "bitmapped". It also cannot be easily reproduced on surfaces like a t-shirt or a hat. The logo can be enlarged as much as is required and not loose its image quality. Also, the vector image can be easily reproduced on all types of surfaces. This is why designers want to create logos that are vector files, not raster files as they allow for unlimited sizes without loosing image quality and they can be used in any application and on any surface.