Unlike most of the other electronic components, the small resistors RepRap uses are non-directional. This means that you don't have to worry about which way you stick them onto a Darwin Power/Communications Card or any other card used by the project; they just work. (Conversely, it is very important for other components to be placed in the proper direction!)
Modern resistors almost all use a standardised method of indicating their resistance, based on a series of colour bands. The colours and their values are shown in the chart on the side, and there are applications on the Internet (linked above) that can translate to and from numerical values and the bands.