Quark is not an engine. Quark is a library which uses SDL2 as engine. It only provides set of commonly used things and utilities which are related to game development. Some of these are: game states, FPS limiter, input interface, resource manager.
Those utilities are kept in an object-oriented model which makes your code cleaner and easier to extend.
See page Getting Started to find out!
Quark engine is still in development, but we already got some testing code. It's just not yet public. Please be patient as we're also working on GLES compatibility...
Quark is available under terms of zlib license. To see more details, check out the LICENSE file(on github)