Inkscape is a graphics editor for vector graphics. Vector graphics means that all the structures are made up from mathematical formulas, and this again means that you can zoom in and out or increase the size of an image as much as you'd like, without having the quality suffer.
Inkscape supports many advanced SVG-functions, like markers, clones and alpha blending.
The software is a socalled "Open Source" project, which means it is made and mantained by people on a volunteer basis. Also anybody can contribute to the project, and this is why it is free both to download and use. If you have Linux, you can find Inkscape in the universe-repository, but if you have Windows or Linux, you have to donwload it using the link underneath.