Inkscape uses SVG as the format for saving its vector files. What is SVG?
The SVG (Scalable Vector Graphics) standard was born at the height of the XML revolution of the late 1990s. In those days, when the lure of simple yet infinitely expressive XML was fresh, people wanted to create XML vocabularies for everything. Vector graphics presented itself as a natural candidate. In 1998, a new working group was formed at W3C, the international consortium that is behind the most commonly used Web standards, including HTML and XML. That group included representatives of (among others) Adobe, Microsoft, and Macromedia. The first fruit of their labors, SVG 1.0, appeared in 2001; the most recent official version is 1.1, published in 2003. The next version, 1.2, is under development, and parts of it are already finalized.
Stemming from the long and often convoluted history of vector formats, SVG tried hard to do things the right way from the beginning. It inherited a lot of good stuff from PostScript and PDF (1.5.1.1 Adobe’s Vector Formats) but was designed to be free from their limitations. SVG natively supports transparency, gradients, Unicode for text, and many other conveniences that are taken for granted in the 21st century. It also adds unique filter effects (Chapter 17), which are basically raster operations (such as blur) that can be applied to an object without losing its vector editability and resolution independence.
Since SVG was designed for the Web, and the most common vector format on the Web was (and is) Flash, SVG also includes quite comprehensive animation features. Version 1.2, still under development, adds more goodies: flowed text, vector effects including nondestructive Boolean operations, device-independent CMYK color, and more.
Currently, all modern browsers support SVG to a varying extent, the only exception being (you guessed it) Microsoft’s Internet Explorer. (For IE, SVG viewer plugins exist, one of which—now discontinued—was developed by Adobe.) This means that you can load any of your Inkscape SVG files into Firefox the same way you would load a JPG or HTML file, and the browser will display your vector graphic exactly as it looked in Inkscape.
SVG is a large and complex standard, and few software supports all of it. Inkscape’s support of SVG is also limited; most notably, it ignores SVG fonts (just like PDF, an SVG document can embed its own fonts) and cannot do animation. Some smaller features are also missing; for example, references to resources (such as gradients or symbols) only work within the same document. It is a stated goal of the Inkscape project to eventually support all of SVG.
When you save an Inkscape document, you have a choice of two SVG formats: Plain SVG and Inkscape SVG. Plain SVG is just that: pure SVG 1.1 code and nothing else. Inkscape SVG, however, adds quite a number of elements and attributes in Inkscape’s private namespace.
Don’t be afraid of Inkscape SVG! It is perfectly valid and standard-compliant SVG; the goal of these additional elements and attributes is just to provide Inkscape-specific metadata about the SVG objects, not to add some incompatible objects of Inkscape’s own. Inkscape extensions may affect how objects behave when you edit them in Inkscape, but they never affect how the document is rendered.[3] Therefore, Inkscape SVG and Plain SVG versions of the same file will look exactly the same in any compliant SVG renderer. The only reason to use Plain SVG may be to reduce the file size or produce a document more suitable for manual editing by an SVG expert.
Inkscape can also save files as compressed SVG (both Plain and Inkscape varieties). Compressed files have the extension .svgz; unlike SVG, they are not human-readable but take much less space on disk. Most programs will read SVGZ files just as easily as they read SVG.