Slic3r is software used to create g-code from 3D models to make them printable on a 3D printer.
The main Slic3r user manual can be found here: http://manual.slic3r.org/intro/overview
Slic3r comes as a stand alone slicing package that is full of useful features, such as setting different layer heights for different parts of a model, or adding modifiers to specific areas of a model for different infill rates. It also comes bundled with Repetier Host controller software, but in a slightly more limited mode. Either way, it is a quite powerful tool with which to prepare and slice models for 3D printing.