The above link mentions the "E" meaning....
In simple terms, this is the chassis/frame. For example, the latest 5 series 2005 & on is an E60. From 1997-2003, the previous chassis (or style) was a E39... Prior to '97, the 5 series was coded E34... and so on... get it? The same applies to the rest of the BMW line up. This helps when you're trying to look for parts.
Is this clear?
What year were you thinking in getting?