Slic3r is the tool you need to convert a digital 3D model into printing instructions for your 3D printer. It cuts the model into horizontal slices (layers), generates toolpaths to fill them and calculates the amount of material to be extruded.
The project was started in 2011 from scratch: the code and the algorithms are not based on any other previous work. Readability and maintainability of the code are among the design goals.
There's serious work ongoing on this software to keep it up-to-date and implement all the necessary features to get a flawless and high-quality printing experience for everyone.