A raster image is the most common type of computer graphic format. Imagine a piece of graph paper in which each square is filled in with a color or a shade of gray. This is a raster grid of x,y coordinates. Note that most computer software cannot recognize raster images as discrete objects; that is why you cannot just select someone's face from a digital camera photo as an object understood as 'face' by the software. To the computer, the raster image is just a bunch of filled-in squares of color. Common raster formats include PNG, JPEG/JPG, TIFF, GIF, BMP.
The other form of computer graphic is the 'vector' format. In this format, shapes are defined by anchor points into lines, arcs, and polygons. Such a shape can be recognized by software as a discrete object, which can be scaled up or down or manipulated in other ways. Thanks to Apple Computer and Adobe Systems, most text is now in vector format, as a series of geometric shapes defined in either the PostScript, TrueType, or Open Type format. That's why zoomed-up text no longer has that jagged, graph-paper look that it did in the mid-1980s.
In an effort to get away from the excessive patent-protections and high license-fees which Adobe uses to profit from the PostScript format, various other vector formats have been developed over the years. TrueType was developed for fonts, and Scalable Vector Graphics was developed for everything else. As I undestand it, Adobe continues to try to block and suppress the SVG format. No surprise: capitalist organizations seek to maximize profit, not to make the world a better place. The good news was that profits made it worthwhile to develop good computer graphics in the first place; the bad news is that exacly the same corporation stands directly in the way of open-source innnovation today.