http://www.techradar.com/news/the-best-free-adobe-illustrator-alternatives

Adobe Illustrator is hugely popular software tool for making vector graphics. Unlike raster graphics (which you’re more likely to use Paint.NET or GIMP for), vector graphics use mathematical expressions to draw the lines, shapes and colours. This means they’re infinitely scalable, so you can blow them up to the size of a billboard with no unsightly pixellation. 

Vector graphics are the best choice for a number of design and illustration tasks, such as creating icons, logos, diagrams, charts, posters and website graphics. In the latter case, the current standard is Scalable Vector Graphics format (SVG).