You start with a model: usually a STL file. People make these themselves or download them from Thingiverse. You then slice the file. This creates a gcode file,

You start with a model: usually a STL file. People make these themselves or download them from Thingiverse. You then slice the file. This creates a gcode file, which is a set of instructions telling the machine how to move and when to print. Gcode instructions are semi-standardized across printers. Depending on your material and your printer, you often need to change the settings in the slicer to get gcode that will create a good print on your printer. You can then use the gcode to print if you printer is connected to a computer. Or you can put it on an SD card and put that in the printer if it's standalone. Some printers (like MakerBots) require further processing of the gcode file into their own formats.