Urban / Rural Definition

Can anyone point me to a concise definition of urban and rural in the 2010 Decennial Census? I have read the Federal Register notices but I am looking for something that is more brief. I am specifically interested in how the population is defined at the block group level. For example, if I look up a block group and it has an urban population of 179 and a rural population of 12, is there a page somewhere that will briefly describe how these are determined?