It seems backwards that MAC says pink is a warm color and yellow a cool, but they use the artists color wheel which is essentially backwards compared to what people normally think of as cool/warm. You can always think of NW as meaning 'not warm', or 'neutralizes warm' and NC as 'not cool' or 'neutralizes cool'.
MAC shades come in level 15 (extra pale) through level 55 (very dark). They make a variety of foundations with different finishes and consistencies to suit different skintypes. It is best to visit your local MAC counter to determine the shade you would wear with the assitsance of a MAC makeup artist. The vein trick written about on the chart is not always the most accurate way of determining your shade because some veins can appear different colors than others. It is best to try the foundation on your skin if you can do so.
Keep it beautiful (✿◠‿◠)