This question has been bothering me since I was six years old- we know that Aunt Buru and Uncle Owen are moisture farmers, but what is it that they do? What is the process? Is it profitable? What is the yield and who buys it, or are they more self-sustaining than commercial? What types of jobs did the droids do?
This is made clearer in the book of the movie - moisture farmers extract water out of the air and sell it to plant farmers.
Since we do not see any vegetation, it may be that there is limited arable land - but water can be extracted from the atmosphere elsewhere and then shipped to where it is needed.
As for process - this isn't detailed in the book, either. You can do this in real life though - moisture will condense on cold metal or rock at night, and you can then collect the dew as water.
Looking at the Lars home and their ability to purchase droids, I'd say there is reasonable money to be had - but we have no real idea of the size of operation they have.
C-3PO was purchased to translate and program the controls of the moisture "vaporators" used to extract the moisture. R2 units have mechanical repair uses as well as their more usual astrogation role.