The source distribution  allows the end user to compile it for any
platform. the binary  distribution is a version that has been compiled
on a target platform. so therefore it may include JNI code. The best
example of this is a Win32 program, such as mozilla. It is compiled
already. But you can get mozilla source  code. If you get the source
code then in theory you can compile a Linux ELF binary.

> It's normally seen that Java based  software like Tomcat, Apache HTTP
Server, Ant etc. all are available in 2 different versions. One is
Binary Distribution and the other is source  Distribution.

--> APACHE HTTP Server is not Java..it's written in C

> I want to know:
> 1. how and why they differ?
> 2. is it possible to write  the same piece of code  using any one of them?
> Your help  is awaited eagerly.

--> Source only contain the source code ... you can compile this
source by yourself. and binary  is the result of source code compilation.
So, If you don't want to get troubled, you should downlaod the binary
one...but if you want to modify the software, you could use the source