Raster graphics map pixels to colours. An image composed in such a way is effectively a raster. The most common file formats are JPG, PNG and GIF. Vector graphics, on the other hand, are defined as 2D points, which are connected by lines and curves (so not necessarily Euclidean vectors) to form polygons and other shapes.
A circle, for example, may be characterised by its center, radius, line width and colour. Only those parameters will be saved, enabling a lossless scalability of the image and requiring often less memory. However, digital cameras produce naturally raster and not vectorised images. Vector graphics are most commonly found in PDF, EPS and SVG file formats.