In the time when the internet was only starting, and once every file designed in Microsoft Word was covered in horrible generic cartoon pictures, there were freely accessible pictures called Clip Art.
The thought behind Clip Art was to offer pictures which can be used freely in documents, without worrying about copyright. This concept was anything invented by Microsoft, and now's somewhat dated. In the year 2009, the internet has become awash with free legal media licensed under what's called a creative commons license. Creative commons is a license which permits original artists to share their work, be it a song, a picture, a video, and have others utilize it under the conditions provided.
That's a good thing as it allows you know where you stand with regard to copyright, which if you are a designers is a superb thing. You don't need your customers to get into trouble over an error you have made with copyright.
Vector graphics are pictures created in applications like Adobe Illustrator or Corel draw. Vector graphics are scalable, and an effective format for pictures which use a lower colour variety, like cartoon illustrations and logos.
Now the cool part is just if you join vector graphics with an innovative Commons license is that you have an entire stack of legal pictures accessible to use for the graphic design projects.
Perhaps you wish to design poster for work, and you're unsure on how to assemble find legal pictures to use, or you need some pictures to use on the company web site, but are unsure where you stand with copyright.