Death Note: Who Exactly Is B?

I%26#039;ve watched the whole Death Note series, I%26#039;ve read some of the manga.. and I read, Death Note: Another Note, The Los Angeles BB Murder Case.


Now I really didn%26#039;t understand the ending and I do plan on reading it again, but I need someone to explain to me if B (Beyond Birthday) was posing as L..? or if he is L%26#039;s brother..? or if L (Ryuzaki), is B.. Or something else and totally different than this..





Thanks to anyone who can help!!!