Clip art used to be clipped from one medium and stuck to another as part of layout design. This clipart caught on quick and spread faster than a Greek army inside the walls of Troy. Free clipart became the new epidemic, infecting designers high and low with its accessibility, utility, and quirkiness.
Vector art is very different from bitmap art which is a photo or an image that can't be resized. Vector by contrast is made up of lines and points, so the program you use to edit it can change its size without getting jagged edges or making the artwork fuzzy. This is especially important if you're creating flyers and posters and want different sizes.
So before you download clipart off the internet, you need to determine what kind you need. For the classroom and basic computer programs such as Word, you most likely just need to get royalty-free raster clipart. If you need to use a high tech computer and software with a vinyl cutter, laser, or CNC then you need a vector clipart graphic. Either way you should use royalty-free clipart to be legal for commercial and other activities.