Raster graphics or bitmaps are a way of representing images. Bitmap images are essentially a 2D matrix of pixels. Pixels (short for picture element) are the smallest units of an image. A pixel contains color values, and together they form an image. A pixel can be stored in different ways depending on what color system we are using.
The simplest bitmap image possible, is a matrix of bits, where 0 would mean the pixel is black and 1 would mean it is white. GIF,PNG,JPEG and most other images with resolution are all bitmap formats. Any sort of image format that has a resolution, or a finite number of pixels is a raster image.
Clearly, rasterized images can only store limited information due to their representation. Vector images can be converted into raster images, and in fact they are. Our screen has a limited resolution and displays rasterized images. Vector images are rasterized to whatever resolution we want.
Related: Image processing