XGA is the terms that describe common resolutions used by computers and projectors as XGA is the main standard of resolution. The resolution is the number of 'pixels' that the projector is capable of displaying and Pixels are the individual dots that make up the image on your computer. Similarly, WXGA is the term that describes common resolutions used by Widescreen (16:9 aspect ratios) home cinema projectors. Every projector has a 'native' resolution (sometimes called 'true resolution'). That's the maximum number of pixels it can actually project individually.
So an SVGA projector can only display 480,000 pixels at a time. In a typical projection screen of 2 meters width, each pixel is going to be a quarter of a centimeter wide, whereas with an XGA projector the image is going to be under a fifth of a centimeter wide, and over 60% more pixels are displayed. This means the image is going to be sharper and less 'blocky' when projecting with an XGA projector. XGA projectors are generally more expensive, and are an equally popular resolution format to SVGA. They have become more popular as XGA resolution computers have become more plentiful.