Slic3r is a slicing software that sets the options for a 3d printer to follow and then creates detailed instructions for the printer by creating a g-code file. This file can be saved to an SD card and transferred to the printer.
Slic3r allows you to arrange, orient, and re-size your design file before printing. It also has option to automatically calculate things such as the top- and bottom-layer thickness and how much in-fill your printed object needs.
Note that Slic3r is not software that can be used to design a model, it is basically a program that takes a final design and translates it into the printer's language.
Slic3r is open source software, anyone can download and install it for free on their personal computer.
Install Link: http://slic3r.org/download
Instruction manual and wiki can be found at http://manual.slic3r.org/