Because they're defined mathematically, vector graphics retain smooth edges at any scale or magnification.
Vector graphics are created in drawing programs that define objects mathematically as a series of lines and curves. Vector graphics are independent of printing resolution. When a printer or imagesetter prints a vector graphic, it marks it on the page according to the mathematical description of the objects in the graphic.
Encapsulated PostScript (EPS) is the most common vector file type. Other vector file types that you can use in Publisher include Windows Metafile (WMF), Windows Enhanced Metafile (EMF), CorelDraw! (CDR), and Computer Graphics Metafile (CGM).
Vector file formats may also be containers for bitmap data. While any vector objects in the graphic will be scalable to any size, bitmaps that are embedded within the file may degrade in quality if scaled larger.