hi now i am download and install ndk but the CDT is important for me...tell me the purpose CDT....
The CDT is for developers who want to do C and C++ development in Eclipse, it's the C/C++ developer tools (see here).
Since Eclipse was primarily built for Java originally (yes, I know its original intent was always to be a generalised environment, but Java really is the "elephant in the room" in terms of what Eclipse does), the CDT is sort of an add-on that allows it to interface to C compilers and other tools.