A website has a special URL which typically points to its homepage. At its best, it is the presence your website makes. The first impression that you only get one opportunity to make on someone. Call it what you will, the design of a website has everything to do with the way your site looks. From basic layout of information and navigation to custom graphics, the design influences how usable and attractive your site is.
Graphic files are the way images get delivered to a browser.
These files tend to be large and are often the slowest part of a webpage load operation. The ones that seem to take forever. The ones that get you so impatient you decide to go to another website. That's a major killer of websites. People get to your website and want something right away.