What is blood urea nitrogen levels in blood urea test?

Hi friends,
The topic of blood urea is irritating me and I am not able to understand that what to do or not. I am not able to get what my book is trying to say about the levels of nitrogen in blood urea test. Even I am not clear that nitrogen is in blood or urea. Because the test is named as "blood urea test" and there is siad to some levels of nitrogen in it.
Every thing is going messy in my head. So, can any one please make me understand that what does this "level of nitrogen in blood urea test means"?
Thanks in advance.

Hi,
Normal BUN levels are 5-18 mg/dL for children; 7-18 mg/dL for adults; and 8-20 mg/dL in the elderly.