Inkscape is an open source vector graphics editor, with capabilities similar to Adobe Illustrator, CorelDraw, or Xara X, using the popular Scalable Vector Graphics (SVG) file format. The SVG format is often used for open source clipart.
The program supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. For advanced users, it is capable of editing nodes, performing complex path operations, tracing bitmaps and much more.
What is Vector Graphics?
In contrast to raster (bitmap) graphics editors such as Photoshop or Gimp, Inkscape stores its graphics in a vector format. Vector graphics is a resolution-independent description of the actual shapes and objects that you see in the image. A rasterization engine uses this information to determine how to plot each line and curve at any resolution or zoom level. Contrast that to bitmap (raster) graphics which is always bound to a specific resolution and stores an image as a grid of pixels.
Vector graphics are a complement, rather than an alternative, to bitmap graphics. Each has its own purpose and are useful for different kinds of things. Raster graphics tend to be better for photographs and some kinds of artistic drawings, whereas vectors are more suitable for design compositions, logos, images with text, technical illustrations, etc.
Inkscape can import and display bitmap images, too. An imported bitmap becomes yet another object in your vector graphics, and you can do with it everything you can do to other kinds of objects (move, transform, clip, etc.).
Source: http://wiki.inkscape.org
The developers of Inkscape are also the founders of the openclipart project, an interesting SVG repository you can use for your Joomla website. More information on the project is available here. For high quality icons, please click here.