Binder is an Android-specific interprocess communication mechanism, and remote method invocation system.
That is, one Android process can call a routine in another Android process, using binder to indentify the method to invoke and pass the arguments between processes.
note that Android does NOT use SysV IPC for interprocess communication
the binder implementation is in the kernel source at: drivers/staging/android/binder.c, with include file: drivers/staging/android/binder.h