THIS description is given in wikipedia [link] about VECTOR
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon(s), which are all based upon mathematical equations, to represent images in computer graphics.

Computer displays are made up from small dots called pixels. The picture is built up from these dots. The smaller and closer the dots are together, the better the quality of the image but the bigger the file needed to store the data. If the image is magnified it becomes grainy as the resolution of the eye enables it to pick out individual pixels. Vector graphics files store the lines, shapes and colours that make up an image as mathematical formulae. A vector graphics program uses the mathematical formulae to construct the screen image by building the best quality image possible, given the screen resolution, from the mathematical data. The mathematical formulae determine where the dots that make up the image should be placed for the best results when displaying the image. Since these formulae can produce an image scalable to any size and detail the quality of the image is only determined by the resolution of the display and the file size of vector data generating the image stays the same. Printing the image to paper will usually give a sharper, higher resolution output than printing it to the screen but can use exactly the same vector data file.

In HUMAN BEING words
vector:small file size (5 to 50 kb) and resolution about 3000x2000.its true you dont believe it (seeing is beliveing) .checkout this file which i have stored in the form of pdf [download] . open this with photoshop and see what you get.

Example showing effect of vector graphics versus raster graphics. The original vector-based illustration is at the left. The upper-right image illustrates magnification of 7x as a vector image. The lower-right image illustrates the same magnification as a bitmap image. Raster images are based on pixels and thus scale with loss of clarity, while vector-based images can be scaled indefinitely without degrading.